A couple of years ago I had problems with keeping it up. I also weighed 300 pounds then and wasn't that interested in sex in general. After losing 100 pounds and cycling this past spring/summer/early fall season between 150 to 200 miles a week at a pretty good clip, I can testify that not only am I hornier than ever, but I no longer have problems with impotence. I'm 47 years old, and cycling/diet/etc has had a drastic improvement on sexual ability.
Sometimes my penis gets numb from sitting for too long in the saddle, that's when I stand up, bounce about a bit, and then continue on. After I got my saddle and handlebar stem dialed in properly and level, the numbness problem is much rarer.
my $.02
Woo