If you have run a marathon, then running a trail 50k will be very similar. The primary difference is your time will be slower. Even hilly marathon courses like the old Mercedes Marathon route had long stretches that were almost totally flat and the climbs, although long were gentle. Trail ultras tend to be quite hilly. Sometimes you actually end up climbing on all fours. This is the case in Mountain Mist in Huntsville where the climb up "Water Line" is a little scary at the top. A slip could send you 20 or 30 feet down the side of a waterfall. Another good example is the Oak Mountain 50K right here in Birmingham. After running down to Peavine Falls from the blue trail, you cross under the falls and climb up the hill to the white trail headed back to the aid station at the Peavine Falls Parking lot. You are pulling yourself up that hill by roots and limbs. Nothing on the Veteran's Park trail is anything like that, but there are several short hills in the trees that will slow you down. The following sections well tell you how you can cope with the various situations and conditions you can expect to encounter in any ultra, even a fairly easy one like The Run for Kids Challenge. You are still running 31+ miles.
Just a little on how to run the race: