Underdown trails are dried out and really fast right now. A pre-ride of the long race course confirmed it at 15 miles. 4 or 5 laps would be possible in the 10-hour race depending on temperatures. The winning 6-hour racer will do 3 laps if it's cool or 2 laps if it's over 80 like today.
Estimates of the short course at 5 miles look accurate, but the loop is all big hills do don't expect to get dizzy from going around and around.