Um, start with small amounts of exercise
If you go out and try to do as many situps as you can, you will have problems. Try to jog a mile or three and ditto. If you sit down to try any exercise program, and go to it with the attitude, "OK! Time to whip this body in shape! No pain no gain! WORK!", you will have a problem.
So don't do that.
Not long ago Greg pointed you at [link|http://www.fourmilab.to/hackdiet/www/hackdietf.html|The Hacker's Diet] and gave his personal testimonial. I read it. I can't give you a personal testimonial from it. But I can say that, the [link|http://www.fourmilab.to/hackdiet/www/chapter1_2_5.html|exercise plan] certainly looks like sound advice. Note in particular that in the "run and jump" phase, "a brisk pace" on his introductory might for you mean a march rather than a run. That is OK.
So try that exercise plan. Your total for the first day is try to bend over twice, do 3 "introductory situps" (much easier than the real thing), 4 leg lifts, 2 "introductory pushups" (again easier than the usual thing), and a bit of marching in place. Try to finish it all in 15 minutes. Not really that much more work than getting in and out of bed a few times, getting dressed in the middle and walking around the block. Repeat the next day, every day, for a week. Then decide whether you are ready to do slightly more or not.
Do that for a month and you will feel like doing more, plus walking down 10 flights of steps will no longer strike terror in your heart. Sooner than you think, walking up them won't be very scary either...
Cheers,
Ben
"... I couldn't see how anyone could be educated by this self-propagating system in which people pass exams, teach others to pass exams, but nobody knows anything."
--Richard Feynman