Seriously, even with just a short flight of stairs, I need to take it slow. But I take that as a sign of progress.
And think stairs are difficult? Try going on the treadmill. I can only do half an hour now (power-walking, no more running), but I also take that as a sign of progress. As long as I burn off at least half of the calories I consume throughout the day, I can be confident that my diet is working.
Today, I ate: 1 bowl cereal measured (150), 1 cup coffee with half & half (20), 1 South Beach fiber bar (110), Lean Cuisine dinner (230); Total of 510. The treadmill told me I burned 266 cals, plus I did arm exercises and sit-ups. That rounds off rather nicely to a total of about 240 calories for the day.