[Endurance] How do I build stamina? I tire from certain unfamiliar movements.

Lately my sex life has picked up, but I've also found myself with more adventurous women. Some positions are more difficult for me to keep up for long periods than others; for instance, if she is on top and riding, it's fine, but the act of driving my hips upwards is unfamiliar and wears me out fast.

How can I build up muscle endurance for these areas, aside from the obvious answer of having more sex?