How do I train my Quarter Horse to get a longer stride in trot?

Hello! On the diagonal, try asking for a lengthening. If it breaks into canter, bring him softly back to trot and repeat, rewarding every attempt at a longer stride. Bear in mind too that a Quarter Horse is bred to go with relatively short steps in trot and to find canter easy, so take care not to ask for something your horse is not physically built to do. That said, being able to vary both the tempo and the length of stride is of course always useful. 🙂