I don't think you need any real level of fitness to do long GA flights.
Of course fitness and exercise enhances brain function, which goes to improved decision making; fitness also contributes to an increase in stamina, again useful for long distance flying.
The business of the PA28 only having one door is really a non issue to me. I happen to have flown a PA 28 180 this weekend and was reminded that there really is no problem. Yes, it could make a difference in a serious accident, but I am not sure it is a significant criteria in choosing an aircraft.