Short. But fun, thick, and with many layers.
What I'm saying is, even if you've only got 20 minutes to ride, that 20 minutes is better than nothing; and sometimes that 20 minutes is better than an all day jaunt. I had a larger, more detailed analysis on that notion forthcoming, part of which surrounds the theory that 3 - 20 minute rides are approximately 2.5 times more satisifying than just one 60 minute ride, but the math on that has gotten a little chaotic, and I dozed off, then woke up staring at half-naked pictures of Danny Devito on the internet, and I lost interest.
What I'm saying is, despite the deficit of time we've created in our lives, ride anyway. You might be dissapointed by only 20 minutes, but probably not. Most of the pyschological benefits are still there, rife with endorphins, poise, and added clarity about life in general. You barely broke a sweat, but the enduring sensation of a rally will stick with you to just about the same extent as a full 2-hour effort. You walk back in the front door, breathe a deep one, buy back into life, and the washing machine dings that the 22 minute cycle you started on your way out the door is complete.
1st place: You, 21:07
2nd Place: Maytag Washer/Dryer, 22:00
What I'm saying is, when in doubt, rally. It might not be perfect, but mountain biking in its most basic sense is a flight from perfection anyway. Get yourself dirty. Slide through the turns.
If only for a few minutes, shake life up, up, up.