Originally Posted by noisebeam
Assuming your otherwise healthy, have somewhat recently seen a Dr...
Get on the bike and ride gently. Keep it in a low gear so its easy to pedal and your legs spin (no hard pushing). Do this for a few weeks and don't worry about training or getting heart rate up to muc, instead get out there for at least 20min at a time and enjoy yourself. After doing this for a while you will get a better feel for your body again and then be a bit more disciplined after a month or two. Of course the approach won't be the fastest way to get in shape, but will likely be the way to do it without injury and something you can sustain while avoiding smoking.
Al
I pretty much second this response. You want to be healthy and fit, not just fit. By taking it easy you will reduce the chances of injury. An athlete with an injury is fit but not healthy.