MTD Mower Stuck in Fastest Speed?

My Lawn Boy MTD 13AO672G120 suddenly shifted into the fastest gear. It made for an exciting ride back through the woods on trails, but I can't get it out of that gear. It starts fine, but even in reverse, goes shooting out of the garage in the fastest gear. Any ideas on a fix? The brakes don't slow it down. The brake spring was gone, but I replaced it with no results.