Chasm54 offered the best advice.

One proviso on Chasm's advice is that one must count ALL exercise into the fatigue / rest equation. If you are totally resting on the off days which means staying off your legs that is one thing. If you are going to the gym, walking, climbing stairs and etc on the rest days, you are taking on additional training load.

Overtraining is more about enzyme imbalances that result is chronic fatigue that takes a long time to get over.

I would suggest taking your HR every morning. If you find it elevated consistently, that is an indication of too much training versus just being tired.