Personally. I do interval training and hill work with my horses. I work up starting with 3 minutes trot three minutes canter 4 times and then increase the pace and length of the canter sets. I also do a gallop set up a long hill or around a track once a week. It's good for the horse's brain and fitness.
This is a fun question. I do gallop sets with my horse! So on ‘off weeks’ (when I’m not at a show), I do gallop sets. This involves opening up my horses lungs to increase his fitness. I warm him up normally but with minimal walking or stopping. I then proceed to canter where I once again start normally but after a couple of rounds on both reins I start galloping. So I’ll have a timer for 10 minutes and within this 10 minutes I have to gallop, shorten his stride, do jump off turns, roll backs, gallop again, flying changes but the catch is that within this 10 minutes I’m not allowed to stop cantering. It opens up my horses lungs, and gets him extremely responsive to my aids. After the 10 minutes is done, I walk for 3 minutes then do another 8 minute gallop set before trotting him long and low. It’s really helped build my horses fitness!