Hey Dodge, this is something I can relate to all too well. Hang in there. It will take time to build up your endurance, but it can be done, of course
Seems like one of the bigger problems is that you mentioned you have been "off and on". This is probably holding you back a bit in terms of endurance. Consistency builds endurance just like anything else. You can't run 10K every month and expect to get any quicker. First I would try to be a little more consistent. Your body will adapt. My work outs from 5-10 years ago look nothing like they do today. 9 sets for back and 6 sets for biceps for ONE workout was all I did when I was starting out, not there is much more volume.
As well, I have never done the workout by Layne which you mentioned, but from what I remember, isn't his program fairly volume? It would seem like even the most dedicated gym goers would have trouble hitting it hard or maxing on lifts through out each workout. Maybe which to a program that has a little less volume. Less volume, more intensity. The HIIT is probably burning you out a lot as well. Especially after workouts.
Lastly, how is your diet and sleep schedule? If you are not eating enough or getting enough sleep each night, this could hurt endurance as well.
I'm sure you have most of these covered, just food for though.