Welcome aboard.
Besides the big scenarios, don't forget to search through the Pfhorums for some short i.e. unfinished but playable ones such as
here and
here. There are others in the Pfhorums.
Also, look
here for youtubes of lots o levels.
There are several channels on youtube that have extensive Marathon levels.
Follow what's going on in the Projects Pfhorum: a new version of the original and a new version of Eternal in the making. You can watch it happen as it happens.
If'n you're home alone and want to play the net maps, take the Simplici7y link on the main page then search for Survival. You'll find a plugin that supplies hoards of enemies.
There are also some good maps on Simplici7y.
And don't forget Yuge. Read the Pfhorum about it's development to understand what it is.
Be patient. Take your time. It's about art. It's about style.
Smooth talkin slow walkin Marathon style.
I just play 'em; I don't know how they work.