Prone ain't no problem
Hang glider pilots fly prone for hours on end. Apparently it takes a bit of practice to get used to it, but it's not a problem. Hang glider flights often last several hours and the longest flights can last ten hours or more.
Prone position is not a serious problem. Power to weight ratio, which depends primarily on energy density of your fuel source once you have to carry 80kg of human, is a serious problem. i.e. battery technology matters. Small scale model aircraft demonstrators don't really prove anything here: there are plenty of model aircraft with amazing flight behaviour. They just don't scale up to carrying a human, that's all.