I'm not in great shape, but I'm not in bad shape.  I've noticed that with Viagra and also with injections, when the time comes for intercourse and physical exertion, I start to lose the erection. Happens when I'm soloing at home, too. If I go mow the front yard or something, whatever medication is in my system doesn't keep me up.

Unlike you, I do not take anything (not judging; just giving you a reference point). Am in my early sixties and in really good shape for our age. And I sometimes lose my erection as well. My guess is that, with age & increased experience, it becomes more of a mental thing than the purely physical.

i try not to over exert the day of the appointment..get some solid rest the day before..and no alcohol..seems to work just fine..

I have been studying erections for a while. My theory for erections to occur there need to be a pressure differential in your penis. When you exercise you expand all your blood vessels equally. So even though you would in theory get more blood into your penis you can also get more blood out.  When an erection occurs you let blood in but restrict the blood flow out. I have observed that it is really  the ability to constrict the vessels at the base of the penis that is the true problem. One of the reason cock rings work is it helps restrict the blood flow out. Even with injection you put the dilator in the cavernosa and let the penis expand but the vessels at the base are still at the same size and unaffected by the dilator. So you have an erection. Add exercise to the mix and two things happen You increase blood flow that washes out the dilator and you also increase your vessel size that lets it out faster. With injectables if you over do it (frequency of use) it seems to be less effective probably for the same reasons exercise inhibits the desired results.  What is interesting (even when young) with exercise your cock gets bigger even in the flaccid state. You probably could help your situation by strengthening your keggle muscles. So I have also observed with all ED meds it is best to relax and enjoy the stimulation that makes you want to have an erection for best results.

I've found myself in the same pattern.  There is a peak of excitement, but I can't maintain that peak for too long.  I've met some providers that are obviously experienced with this, and they guide me right into the appropriate activity in a timely fashion.