Its easy to hurt yourself with any excercise, but if you are sensible and ride regularly, moderately, and for a resonably long time, aerobic fitness will creep up on you without busting too many joints.
You should be able to ride for a few hours at a steady pace, without getting into a huff. If it hurts, then use a lower gear. You dont have to ride in the same gears as a young athletic racing cyclist. Do you have low touring gears on your bike ?

Easttexan;
On your original question about wind, you will find that for the most part it is a matter of overall body fitness. If you remember that right after your bypass you were puffing like a freight train just walking to the bedroom and back.

As you increase your mileage and your riding speed, the hill that you used to run out of wind on is now a slight grade that you do not even downshift for. At one time riding 5miles was an accomplishment for me and now I do not even get the kinks out of the old body for 5 and don't considerate it a ride unless it is at least 25 to 30.

Now of course all the above statements about my conditioning and me are dependent upon me surviving this series of heart attacks. And I know that my reconditioning will include a whole lot of huffing and puffing up hills that I never realized were there before.

*looks around, confused... then re-reads title*

Oh, sorry. Thought this was a thread about <em>breaking </em> wind...