Based on the comment by Tadaen Sylvermane, I realized there was something sort of corrupted on the originally-listed sources file. I decided to replace that with a default sources file for my distribution. I went to: https://repogen.simplylinux.ch/index.php, selected the default repositories for my country and distribution, generated a fresh sources file, replaced my old file with the newly generated file, and executed an apt update.
As shown, some URLs were still missing:
ejspeiro@Eduardo-Alienware-14:/etc/apt$ sudo apt-get update
Ign file: InRelease
Get:1 file: Release.gpg [819 B]
Get:2 file: Release [574 B]
Ign file: Translation-en_US
Ign file: Translation-en
Hit http://ppa.launchpad.net trusty InRelease
Ign http://us.archive.ubuntu.com trusty InRelease
Hit http://ppa.launchpad.net trusty/main amd64 Packages
Get:3 http://us.archive.ubuntu.com trusty-security InRelease [65.9 kB]
Hit http://ppa.launchpad.net trusty/main armhf Packages
Hit http://ppa.launchpad.net trusty/main i386 Packages
Hit http://ppa.launchpad.net trusty/main Translation-en
Get:4 http://us.archive.ubuntu.com trusty-updates InRelease [65.9 kB]
Get:5 http://us.archive.ubuntu.com trusty-proposed InRelease [65.9 kB]
Hit http://us.archive.ubuntu.com trusty-backports InRelease
Hit http://us.archive.ubuntu.com trusty Release.gpg
Get:6 http://us.archive.ubuntu.com trusty-security/main amd64 Packages [588 kB]
Hit http://us.archive.ubuntu.com trusty-security/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty-security/universe amd64 Packages
Hit http://us.archive.ubuntu.com trusty-security/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty-security/main i386 Packages
Hit http://us.archive.ubuntu.com trusty-security/restricted i386 Packages
Hit http://us.archive.ubuntu.com trusty-security/universe i386 Packages
Hit http://us.archive.ubuntu.com trusty-security/multiverse i386 Packages
Hit http://us.archive.ubuntu.com trusty-security/main Translation-en
Hit http://us.archive.ubuntu.com trusty-security/multiverse Translation-en
Hit http://us.archive.ubuntu.com trusty-security/restricted Translation-en
Hit http://us.archive.ubuntu.com trusty-security/universe Translation-en
Hit http://us.archive.ubuntu.com trusty-updates/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/universe amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/main i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/restricted i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/universe i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/multiverse i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/main Translation-en
Hit http://us.archive.ubuntu.com trusty-updates/multiverse Translation-en
Hit http://us.archive.ubuntu.com trusty-updates/restricted Translation-en
Hit http://us.archive.ubuntu.com trusty-updates/universe Translation-en
Hit http://us.archive.ubuntu.com trusty-proposed/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/universe amd64 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/main i386 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/restricted i386 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/universe i386 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/multiverse i386 Packages
Hit http://us.archive.ubuntu.com trusty-proposed/main Translation-en
Hit http://us.archive.ubuntu.com trusty-proposed/multiverse Translation-en
Hit http://us.archive.ubuntu.com trusty-proposed/restricted Translation-en
Hit http://us.archive.ubuntu.com trusty-proposed/universe Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/universe amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/main i386 Packages
Hit http://us.archive.ubuntu.com trusty-backports/restricted i386 Packages
Hit http://us.archive.ubuntu.com trusty-backports/universe i386 Packages
Hit http://us.archive.ubuntu.com trusty-backports/multiverse i386 Packages
Hit http://us.archive.ubuntu.com trusty-backports/main Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/multiverse Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/restricted Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/universe Translation-en
Hit http://us.archive.ubuntu.com trusty Release
Hit http://us.archive.ubuntu.com trusty/main Sources
Hit http://us.archive.ubuntu.com trusty/restricted Sources
Hit http://us.archive.ubuntu.com trusty/universe Sources
Hit http://us.archive.ubuntu.com trusty/multiverse Sources
Hit http://us.archive.ubuntu.com trusty/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty/universe amd64 Packages
Hit http://us.archive.ubuntu.com trusty/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty/main i386 Packages
Hit http://us.archive.ubuntu.com trusty/restricted i386 Packages
Hit http://us.archive.ubuntu.com trusty/universe i386 Packages
Hit http://us.archive.ubuntu.com trusty/multiverse i386 Packages
Hit http://us.archive.ubuntu.com trusty/main Translation-en
Hit http://us.archive.ubuntu.com trusty/multiverse Translation-en
Hit http://us.archive.ubuntu.com trusty/restricted Translation-en
Hit http://us.archive.ubuntu.com trusty/universe Translation-en
Err http://us.archive.ubuntu.com trusty-security/main armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-security/restricted armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-security/universe armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-security/multiverse armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-updates/main armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-updates/restricted armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-updates/universe armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-updates/multiverse armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-proposed/main armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-proposed/restricted armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty-proposed/universe armhf Packages
404 Not Found [IP: 91.189.91.26 80]
Err http://us.archive.ubuntu.com trusty-proposed/multiverse armhf Packages
404 Not Found [IP: 91.189.91.26 80]
Err http://us.archive.ubuntu.com trusty-backports/main armhf Packages
404 Not Found [IP: 91.189.91.26 80]
Err http://us.archive.ubuntu.com trusty-backports/restricted armhf Packages
404 Not Found [IP: 91.189.91.26 80]
Err http://us.archive.ubuntu.com trusty-backports/universe armhf Packages
404 Not Found [IP: 91.189.91.26 80]
Err http://us.archive.ubuntu.com trusty-backports/multiverse armhf Packages
404 Not Found [IP: 91.189.91.26 80]
Err http://us.archive.ubuntu.com trusty/main armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty/restricted armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty/universe armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Err http://us.archive.ubuntu.com trusty/multiverse armhf Packages
404 Not Found [IP: 91.189.91.23 80]
Ign http://us.archive.ubuntu.com trusty/main Translation-en_US
Ign http://us.archive.ubuntu.com trusty/multiverse Translation-en_US
Ign http://us.archive.ubuntu.com trusty/restricted Translation-en_US
Ign http://us.archive.ubuntu.com trusty/universe Translation-en_US
Fetched 785 kB in 5min 41s (2,302 B/s)
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-security/main/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-security/restricted/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-security/universe/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-security/multiverse/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-updates/main/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-updates/restricted/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-updates/universe/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-updates/multiverse/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-proposed/main/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-proposed/restricted/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-proposed/universe/binary-armhf/Packages 404 Not Found [IP: 91.189.91.26 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-proposed/multiverse/binary-armhf/Packages 404 Not Found [IP: 91.189.91.26 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-backports/main/binary-armhf/Packages 404 Not Found [IP: 91.189.91.26 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-backports/restricted/binary-armhf/Packages 404 Not Found [IP: 91.189.91.26 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-backports/universe/binary-armhf/Packages 404 Not Found [IP: 91.189.91.26 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-backports/multiverse/binary-armhf/Packages 404 Not Found [IP: 91.189.91.26 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty/main/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty/restricted/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty/universe/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty/multiverse/binary-armhf/Packages 404 Not Found [IP: 91.189.91.23 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.
However, when executed the installation of aptitude, it all went great. Furthermore, the update system starting prompting me for some updates now.
As far as to why https://repogen.simplylinux.ch/index.php generates source files with outdated URLs baffles me. Furthermore, I do not understand why, if this is an LTS which is supposed to be supported until 2019, then why are repositories that could not be found?
Anyway, I am satisfied with this procedure and its outcome, which is why I decided to post it as an answer to my original question.