I agree with DSDude, everyone should watch their fitness. Fitness has less to do with weight than many (perhaps most) people think. In a general sense, it is about physical capability.
Physical fitness - Wikipedia, the free encyclopedia
I'm a tad underweight (about 143lbs/6'2") and I'm far from fit. I went biking on Tuesday, 12-14 miles, for the first time in about 4 years, and I could barely stay above 10MPH most of the way and kept falling behind the other 2 guys I was biking with. I plan to go biking every week or two because I obviously need the exercise.