5K

It is the distance that requires the perfect mix of raw speed and endurance;
the distance that the average runner can easily test their fitness or progress;
sucess at other distances doesn't guarantee sucess at this distance - the miler is too short focused and the marathoner is too long focused.
The best in the world cover the distance in under 13 minutes; anyone who covers the distance in running shoes has accomplished something;
To the American, the distance is odd, 3.1? To the rest of the world it is an even 5000 meters.
Just as the first mile adrenaline fades, the middle mile is upon you and the last mile is as quick and painful as you want to make it... but then it is over.
The greatest American runner* ran this distance. To paraphrase him, You have to run every race as hard as you can, to do anything less is to waste the gift.
He also said "I don't race to see who is fastest. I race to see who has the most guts."

5000 meters, 5K, 3.1 miles. It is a great distance.
5000 also happens to be a great fundraising milestone.

*Steve Prefontaine