Sex and Physical fitness?

Hello.. i am not overwight, sometimes I work out regularly but not lately - in general I'm in good shape. Also not very experienced with sex and it takes me a really long time to come, if ever. But basically, I didn't know it takes so much physical fitness to be on top! How is this possible?! I thought I was in good shape, lol.. also, any advice on how to come vaginally? My guy feels bad that I never quite make it, lol.