Sex help..

When my boyfriend and I have sex, he constantly does 95% of the work. Every time. We both enjoy it but I know he would like it if I actually did stuff too. The only problem is that when I ride him, I can't do it for very long because I'm so out of shape. I'm very skinny and a young adult but you would be shocked how out of shape I am. It makes me tired to walk for a long time, go up and down stairs, and running isn't even possible unless it's for like 3 minutes. Anyways though, I was wondering if there was away to be able to ride him longer. Any techniques that help not make me so tired? Or should I do a certain workout everyday to build riding muscle? 😂 Please help.