Honestly if the SP's you're booking say ''I can't even walk anymore'' that is not a good sign............ Booking a longer session will not solve the problem. We are not
sex dolls or machines.. Some of us can handle more than others however, but if this is a pattern, if I was at your place, I would question myself.
I don't think what you're looking for is a GFE session. Maybe look for someone who offers PSE and clarify that you are going to want to have sex for the whole booking. Some SP will accept, some will not. The idea of booking a duo is good too