if you don't regularly go to the gym

[JeffreyLebowski]

How about your core? Situps only train a small part of your abdomen. Stuff like squats strengthen your entire core everywhere instead of small areas. It helps develop the stabilization muscles that are key to good core health and strength.

Some people take it waaaay to seriously. If what someone does keeps them feeling good and not fat, it's non of your business. I'm 5'10" 170lbs and only work out once or twice a week. I haven't run into anything I can't do in my day to day life that would require me to go to a gym and work every muscle in my body.
Cardovascular health is the most important aspect, working out all your individual muscles to peak capacity is for people with self image problems.

 

[Saint Nick]

working out all your individual muscles to peak capacity is for people with self image problems.

Or to relieve a stress-laden life

 

[Lamont Burns]

Cardovascular health is the most important aspect, working out all your individual muscles to peak capacity is for people with self image problems.

That's an impressive amount of stupidity in one sentence.