Keep up the good work. I can relate my experience.
At 48, I started back lifting while taking both a statin drug and an anti-hypertensive. I was also overweight. I became tired very quickly, especially in the upper body (shoulders), as a result of not only my lack of conditioning, but also the meds. I stuck with brief workouts, but remained consistent, alternating them on a daily basis with a HIIT-type stationary bike session (also brief).
Over time, my conditioning improved. I lost weight, grew stronger, and have been able to do more. As a result, I have been able to halve both the Zocor and the BP med. Maybe I could go off entirely, but I have genetic predisposition to cardiovascular disease, also.
Now, 48 and 76 are not the same basis for a prescription, and I am by all definitions an amateur. That said, I would recommend shortening your workouts, sticking with perhaps only 2 or 3 compound exercises. That way, by the time you're tuckered out, you would already have done the most important stuff. On this site, read all the Dan John stuff, it's helped me a lot.