Robin, right now I'm just walking, outside. No dreadmill for me. When not in pain I usually walk a 15 minute mile. Not too bad for a short, stubby guy I guess. I think the only requirement for the 1/2 is to finish it within 4 hours. I doubt that I'll be able to get under 3 hours this year.
I usually walk/run. I can't run far before my heart is about to explode and jump out my chest (literally). About a 1/4 mile at a time is it for me. In the past I've tried to walk the first 1/2 mile to warm up and then jog a short distance, walk a nice stretch, then jog again. If you've done a triathlon, a 1/2 marathon should be a walk in the park.