A good rationale for the three-week taper:

My feet, ankles, shins, knees, hips, glutes and shoulders are on fire 24 hours after my last 20-miler. In three weeks, I can regain the spring in my step while training in reduced amounts. Two weeks? I'd be afraid. Very afraid that I'd not start the marathon on fresh legs. Right now, I run like a true geezer.

I'm starting to think my next avocation will be bowling in the winter and golf in the summer. I know I can go sub 300 in bowling...and still walk down stairs the next day.