I'm sorry, but even if the OP is moving at 105% all the time, he simply cannot "overtrain" riding 12-20 miles per day (in two sessions no less.) Some days the energy level isn't there. It happens. Overtraining is a real thing, people, and it's not just a "Oh, I didn't feel as fast today." It's a diagnosable condition, and you don't want it. It's getting kinda tiring seeing it thrown around so often by the 60-miles-a-week crowd.

I ride every day, averaging over 35 miles per day. That's about 260 miles a week. Yesterday was I think Day #44 without an off day, and my energy level was down a little-- so I only did 74 miles @ 18.4mph. The first hour my legs felt like molasses. Never once did I suspect the fearsome overtraining. I was just tired. I was stronger in the last hour than I had been in the first.

Keep at it, OP. At your miles per week, fitness/health permitting, you could ride seven days a week forever.