I have foot/leg problems, too, though not so severe that I can't ride. You're not very specific about what you want or need, so it's hard to give precise recommendations. A few shots in the dark:
Tires make a huge difference in a bike's feel and performance. Something like a Panaracer Pasela in a fairly large size (I use 700x37) would be light, roll well and still have enough volume to cushion the ride.
Sometimes, just more time in the saddle will help. Maybe not in your case, but for me, small increases in time and distance over several months got me from struggling to do 10 miles to riding 25 with ease.
Is your position right? Fiddle with the seat and bars to see what works. The "correct" setup for racers doesn't work for everybody.
Have you tried different pedals and shoes? They don't have to be bike shoes and expensive pedals. After 30+ years of riding, I've gone back to old-style pedals with toe clips and loose straps, with running shoes or lug-sole mountain bike shoes without cleats. Bonus: the change didn't slow me down at all, and I can stop for coffee without walking like a duck.
Recumbents are also a good suggestion. I've looked at them and may wind up with one eventually, but it's hard to do for less than $1200-$1500.