Yesterday I tried to jog following a 19 mile day, and I could barely get moving. Clearly when your legs are that broken down any running you do will be counterproductive and you are better off staying on the couch for more football.
Don't forget, training is a continuous cycle of tearing down and rebuilding (recovery). The greater the damage to muscle tissue, the more time required to bounce back. The average runner sometimes needs to experiment with different workout/recovery ratios to see what works best. When I am in good shape, I've found that I need one day to recover from an average 2 hour run, so I am able to perform a quality workout about 48 hours later.
What should you do on the recovery day? This is where I like to plug in a bike or some other workout to give my cardiovascular system a workout while my legs recover. I've found that leg muscles beaten down after a long hard run feel surprisingly good when I get on a bike.
On the occasions when you go too far on your long run, you may even need 2 days or longer to feel normal again. If you don't like taking 2 full days off, one trick you can try to shorten the length of your recovery is do your long run in the morning, then after your recovery day wait until evening of the following day to try your next workout. An extra 10 hours of recovery and nourishment can make a big difference, and sometimes you can still get in a shorter quality workout on one day recovery.
Finally, I've often found that while the first 1 or 2 miles back feels terrible, the longer I warm up the better I feel. Give it a chance, and you may find your legs feeling more recovered than you thought.
(c) Dave Elger, 2009 All rights reserved