The Marlborough downs challenge I have done it twice now and a couple of other short ultras as well. 33 miles was fine, not significantly more difficult than a marathon. Marlborough downs is a bit flatter than Stenying and in the run up to it I was running marathons every month so I was used to doing 26.2. The mistake I made/ always make is setting off too fast. What sort of triaing have you being doing, back to back long runs or really long runs?