Welcome to the site Pony.
All the modifications that have been mentioned will help.
The weightlifters belt may help. but I think it misses the point just a little.
The best modification you can make is to yourself. Get thee to a gym and start a lifelong routine of fitness. Cardio exercise and weight training. Strengthening your muscles will do wonders for your ability to ride for long periods of time.
I'm 53, 6'4" 260 lbs and do just fine with the Speedmaster.
I doubt that you will find anyone on this board who has a fitness routine that has the problems mentioned.
In fact, I think I'll start a thread on the subject
