A Fat Ass run is basically just a slightly structured community run. The motto is "No Fees, No Awards, No Wimps."

I think it all started with some guy in California being mad there was no ultramarathons on New Years Day so he threw a "Burn Off Your Fat Ass 50 Miler" as a no-aid point to point run to work off all the holiday calories, or something like that.

The way that the Trail Monsters put on Fat Ass events is we basically come up with a loop, usually on trails (it is a trail club, after all) and while there are no fees it is encouraged that everybody bring some food for the aid station.

Aid stations have ranged from the back of Erik's pickup truck to a fully stocked kitchen. 

And we like bringing food out. I usually bake a loaf or two of bread, somebody usually brings soup (we had 3 different kinds all heating in crock pots this weekend), there's always bags of candy or various energy drinks and products (these are a good event in training to see what your stomach can tolerate), plus there's various cookies and cakes and such. We even had baked mac & cheese at hte last one.

People can run as many laps as they want.  This weekend, I had an 8 mile loop with a 5 mile cutoff for those that didn't want to go 8 miles, so some people did 1 5 mile loop, some people did 1 8 mile loop, some did an 8 mile loop (or 2) followed by a 5 mile loop, some did 3 8 miles loops, and I did 3 8 mile loops plus the 5 mile loop with a 2 mile out and back near the end so I could get up to 31 miles.  (We almost always make them "officially" 50k although this is the first time I went the full distance.)

Here are a few writeups of various Fat Asses that we've put on over the past couple years:

http://www.1moremileforsunshine.com/377/day-31-a-strong-finish-31-miles-for-me-258-miles-collectively

http://news.runtowin.com/2008/03/02/trail-monster-running-fat-ass-50k-2008.html

http://news.runtowin.com/2009/03/05/so-whats-a-fat-ass-race-really-like.html

http://trailmonsterrunning.blogspot.com/2009/02/tmr-fat-ass-50k.html

http://trailmonsterrunning.blogspot.com/2010/01/gac-fat-ass-marathon.html