[watwut]

If you mistake the soreness and fatigue for pain and stop at that moment, you will still get in better shape. Really. It just gonna be slow. If you mistake pain for soreness and fatigue, you wont exercise until that injury you just caused yourself heals.

Also, if you are overweight, did not exercised for long, but are ambitious, that is the prime situation where it is easy to cause yourself an injury. Fairly fit means that your body is adjusted exercise at least somewhat and joints and such can withstand.