As a good downhiller, I like the idea of downhill races. It would diversify racing in a direction similar to mountain running, which has both uphill races and up and down races. But I think it should be on a more ridiculous kind of course, so that there is no confusion about what it is. As a side benefit, you don't need world class marathoners to do it, so talk of ruining real marathons by drawing away talent would be moot. It should be lower key, like Big Wave's (Dave Torrence's) downhill mile.
I could see myself having fun in a marathon or half marathon down a road like Haleakala (10,000 ft in 35 miles). I'd prefer the half marathon myself, having never raced well beyond about an 1 hour 45 minutes. I think that even at 47, and never all that fast when I was younger, I could probably go 1:02 in a half marathon down a road like that. There are probably 2:20-2:30 marathoners that could go sub-2:00 marathon on a road like that.
Following up my thoughts, it would be great if Letsrun messes with Nike and beat them to a sub-2:00 marathon by holding a flash-mob free marathon on a downhill like Haleakala.
It's similar to my idea to offer money for a female sub-4 mile.
How much money do you think we need to raise on one of those funding sites for us to get someone to break 2:00?
Hell, we might not need to raise any money as we could probably get Red Bull or someone to pay for it. But how much money do you think it would take?