I'm 44, started training at 42, and have a desk job. I see these types of posts once in awhile about training before/during training. Since I've been thru it already, my suggestion is always, "show up and do the best you can". The workouts will take care of everything else in terms of cardio, strength, etc.
Don't compete physically with the 20/30 somethings, you're 40. Training this stuff is an accomplishment on its own. Be proud of that. Personally I've found that getting as technically proficient as possible has become my "edge". While I would kill for their cardio., I've found that technique goes along way towards evening the odds.
Also, what you have been told about pacing yourself is spot on. Learn to relax and do things correctly without resorting to pure strength. You'll last longer.
Feel free to message me if you need other help/suggestions. Us old guys gotta stick together!!