If you and your horse can do ten miles 2 days in a row at a decent pace ( 6 to 8 mph) then you can get him through a 30 mile ride. I know from experience. As long as he has been being ridden, he will be fine. By no means will you be able to run up front, but to finish just about any horse being ridden regularly can finish an LD. If a horse will take care of themselves during the ride ( eating and drinking) they will do just fine. Good luck! Endurance is an awesome sport and I wish more people would join in on it. Depending on where you live you coukd find a mentor that does endurance.