Exercises for stamina?

I would like to increase the amount of time I can perform piv for my partner's in more interesting and taxing positions however I find it difficult to maintain a good rhythm and my erection for extended periods for two reasons. My cardiovascular health seems like it could use some work which I more or less know how to do, the other problem however is that many fun positions take a lot of muscle use which inevitably seems to drain blood from my erection, is this problem related to cardiovascular health? If not strongly related is there anything else I could consider that might help? Thanks all