There's a race in Manchester that bills itself as the fastest race you'll ever run, as it's all down hill, but I can't get very excited about it. I wouldn't win, of course, and it's barely enough time to work up a sweat. Maybe if there's a great party afterwards or if a bunch of friends want to do it. So 5k is it for me.
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Depends on how far away it is, what the total cost of the race is (entry free, transportation, hotel, food), and what the awards are for the top finishers.