In retrospect, I think the reason the Waterfall run is so hard is because it is all up and then all down. There is not a lot of rolling hills. It reminds me a lot of the infamous 1/2 marathon in Boise called the Race to Robie Creek which is eight miles of uphill followed by five miles of intense downhill. Unlike Robie, waterfall has equal miles uphill and downhill but like Robie, each is equally hard because having your body get pounded on the downhill after so much climbing really takes its toll on you.