How is exercise and fitness related to one's sexual performance?

Stamina. More oxygen to your heart and brain mean greater stamina and enhanced awareness of everything around you. Also, greater endorphin release means heightened pleasure and appreciation of what's happening to you in the moment.
Lots of ways. Better fitness leads to better stamina, which leads to better performance. Better fitness leads to better blood flow, which means that the sexual organs are better supplied with blood and oxygen, which they need to function properly. Better fitness improves self image, which has a lot to do with how attractive others find you. The list goes on.