"... as fast as your legs can carry you." at least works and sounds natural enough I guess. I don't think anyone would say "feet" here instead of legs. As for your #2, it sounds like a medical condition. From our point of view---or at least from mine---the "leg" includes the foot as a part of it. He could have trouble with his feet or with his legs. It just depends on what medical condition he has, where it is located. Either one could be true.