For fitness I don’t think you can beat walking. But not slouching along. Marching is the order of the day!
I personally think old school walking hacking, like they used to do with hunters, is under utilised. My personal experience comes from rehabbing a ligament injury three years ago, so I did SO much research/discussions with my vet/looking into fitness programs. I had the extra struggle that before I started my horse didn’t hack alone (but was brilliant in company). He now happily hacks alone up to 13miles and competes in low level endurance alone up to 20miles.
So personally I would do 6 weeks of walking hacking before anything else (assuming you’re starting from scratch or thereabouts, that your horse has no medical/soundness issues etc).
I would build up from doing a mile long loop (which should take you just under 20 min), to several miles, walking for 1.5-2hrs/5/6miles. Use any hills that you have, but introduce them slowly- both up and down are hard work and require core strength. If your horse struggles on the steep downhill to start, weave from side to side, rather than straight down. If he’s under confident or likely to nap, make sure your routes are circular or lollipop shaped, do not turn around and go directly back on yourself!
Everyone has different opinions on this, and horses are all different, but personally mine was more confident with a foot soldier when starting alone. Now, if you have a willing helper, they can walk with you (easy at the start as you’re only going short distances), and then increase the distance between you and them, so the horse is ‘alone’ gradually. If you don’t have a helper, start off with you walking, probably on the way out, when you’re horse is worried about leaving his friend, and then hop on on the return when they feel more confident, over time reduce the time you’re on foot, getting on board earlier and earlier until you can ride out of the yard.
If you are walking out/marching, it really will help his fitness. For example, we have a lot of steep hills near us. My horses heart rate can go up to 120 walking up v steep hills. That’s the same as a long canter on the flat for him- so it’s improving his fitness without putting any extra strain on his legs. Walking up hills uses more muscle strength than higher paces as they don’t have the forward speed/motion to help carry them. If (during the last few walking weeks) you want to make the exercise harder, put some halts in as you go up the hill.
Hopefully, by the end of the 6w your horse is feeling more confident in your abilities together and you will be able to ride straight out. At that point, you can start introducing trot. I start with 2 mins split up over the course of a ride (so a 15 sec burst, then 15 min later maybe a 30sec burst, etc etc). Every week add an extra 2 min, till you’re doing 15 min trotting over the course of a ride (that Will be about 2 miles of trotting). At that point, personally, I start being less strict and might then increase it by 3/4min the next ride and so on, as the % increase is less compared to the overall.
At that point (hopefully!) your horse will be feeling really good in himself, and finding the above easy. You can then start introducing short bursts of canter. Again 15 sec or so. Don’t aim for 1m to start with, s/he’ll tire and end up on the forehand. Gently increase it so he finds it easy. Lots of small bursts coming back to trot early whilst he still finds it easy, rather than trying to fit it into one block.
The ‘rule’ is to not increase your speed and distance at the same time, one or the other, mix it up over different rides.
Once you’re at the point where you’re both comfortable and happy hacking for a few hours, with 3 miles of trot and 1mile of canter in your hacks your horse will be fitter than the average ‘leisure horse’. You will then have a really solid base to add skill work in, eg schooling/jumping etc and you will have lowered his risk of injury. To start, it’s slow and takes a long time, but I think it’s worth it for the longer term benefits.
I would highly highly recommend a tracking ap. I use Equilab but there are several. You can then see what you’re actually doing- when I started I was vastly overestimating!! Eg you’d think you’d had a lovely long canter stretch, you’d done loads and he’d worked hard….. but when you looked at the stats later, it was only 43seconds!
As you build up your trots and canters keep an eye on his HR. Obviously a wearable monitor is ideal but not actually necessary. The HR drops quickly (or should do if you’ve built fitness up slowly). But you can always have a little look when you finish a long ride, check his pulse by feeling for it under his jaw. Get your phone timer on for 15 seconds and then x4 his pulse. As a guide it should be under 64bpm within 30 min or he’s been pushed too hard and you need to dial it back.
Having put the work in my horse has so much more confidence in us/ me and my ability to keep him safe and make good decisions for the both of us. And we were starting from a good place! (I think!!!) I’d already had him 7 years when he did his injury!! He’s now, in his late teens, stronger and fitter than he’s ever been and we are a much stronger partnership.
good luck!