I have this "problem" as well...
I feel I'm underweight abit, and have a hard time gaining. My diet isn't a strictly monitored one, but I eat somewhat "regularly" and "healthy". I do "eat-out" too (3-5 meals/week)...
I'm 5'10", 140lbs... My BMI is 20, within the 20-25 "normal" range (which for my height is between 140-175lbs). And I smoke, but want to quit.
* for shits-n-giggles I tried doing some push-ups and sit-ups tonite, no practice just do as many as I could consecutively...
P-ups = stopped after 25, and able to do 10-15 more no doubt...
S-ups = hesitated after 6-7, but got the knot out of my stomach and pulled off 25 with some gas left...
** I also pulled up google-maps and mapped a perfect 2.4km route from my doorstep. I'll try it tomorow...