i rode one (dax 70) 165 miles in one day. averaged about 27mph with a heavy load. stopped every hour to let it cool down and take a break myself. the only mishap was a flat rear tire due to my failure to spot pre-existing damage.
now that you know that, i'll say this (a 2-pt personal opinion)...first, if you average 35mph (the top reasonable expectation) for 500 miles, you won't have to worry about repairing it, you may well be replacing it. second, i personally wouldn't buy a 2-stroke HT if my primary goal was long-distance running...permanent damage has a way of sneaking up on you with the little bangers.
huh, i guess both parts said the same thing in different ways...oh well.