Welcome Carstn davidwendel21!

David, you might get more responses if you start a new thread, but the short answer is sounds like to me you're using muscles that have forgotten what bike riding is all about and you just need to give yourself a chance to get used to riding again. Start out at short distances and work your way up. You'll be surprised at how quickly you'll get better.

FWIW, I lost close to what your doc says you need to lose over a two yr period and it took six months of walking before I even got back on a bike.