Yes you can swap to a compact crank, if you look at the front derailleur you'll see that it can be adjusted up and down. It's a good idea. Nobody really needs the top end gears and the lower ones are really useful.
Can't give advice on your asthma, if it is inhibiting your riding you need to talk again to your doctor. But I do know that there are international athletes who are asthmatic.
The best first step is...the first step. Don't worry about not being able to do what you could do before, just start by doing what you can and persevere. The good news is that having been there before, you (and your muscles - they will remember) know you can get there again. Asthma apart, you'll be surprised at how quickly your body responds to a resumption of exercise. Set very small goals, initially about riding time rather than distance. Build up 10% per week, so if you ride three hours this week, ride three hours twenty minutes next week and so on. Only after a few weeks of this would I start to think about moderate increases in intensity. Speed, and distance, will follow in time.
As you see some improvement you'll be more motivated to continue, and to want to take the trouble to eat properly- which is still the most important thing as far as the weight is concerned. And you are still a young man, in a year or so you'll be asking advice about training for racing.
Last edited by chasm54; 08-08-11 at 03:22 AM.