In the Staying Fit section I talk about starting small and adding time/intensity as you go. Although you will have to push yourself some (you'll never improve by sitting on the couch), don't push yourself if it feels too much. How much is too much? Well that depends on the person. What is too much for me might not be too much for you, and vise versa.
For example, my fiance is 6'3" and makes running look easy. If I try to keep up with him I'll give out, durn those long legs! But if I set my own pace (meaning make him do laps and come back for me), then I can run much farther and push myself at my own rate.
So... push yourself so you get a good sweat and get your heart beat up, but at the same time if you ever feel light headed or weak then tone it down. Like my brother pointed out, "Rome wasn't built in a day after all".
Love you Ryan. :-)