Right off the bat I think it needs to be said that snowboarding is largely a technique driven sport. Some high level riders (like Ryan Knapton) are admittedly "too fat and not strong enough", but because their technique is so good that they use much less muscle force to accomplish the same thing.
By riding more you get better, specifically in these ways:
- You're more comfortable going faster, so you don't push your edges so hard.
- You can ride your board more and not fight it as much.
- You don't crash as much, and those crashes really can make you sore (or worse).
- You ride the appropriate board for the conditions so as an example you won't be trying to butter/tail press a small board through deep powder and will rather just ride a bigger board.
- You can ride switch to give your back leg a vacation and share the load a bit.
None of that has anything to do with squats or box jumps, rather it's just the benefit of riding all the time. Those muscle memory skills tend to stick around for years (even decades) unlike muscle fitness which can drop within weeks if not used.
But even for an accomplished rider in good shape, something like pushing through deep powder all day long will exhaust you. Carving your way down blue groomers is one thing but ruddering around your tail in powder or blasting through tracked out lines is much more energy demanding. To that end, these are the training techniques I employ and would encourage others to do the same:
- Back squats. The head-to-toe nature of these are just too good to ignore. I would focus on the higher rep range (8-12) unless you have other goals.
- Balance board stands and balance board squats. This encourages good technique which reduces the muscular force employed.
- Wall shots. I like these in particular because they teach explosive (power) moves, especially if you get your feet to leave the ground on the toss. But on the catch you end up being slightly cock-eyed and have to adjust quickly which is common on a snowboard.
- The seated ball throw is good, I use a medicine ball with some bounce and throw it against a wall (~3 feet / 1 meter away).
Just on a personal note, I live at 8K feet in Mammoth, and put over 120 days on my season pass last year. But a lot of those "days" where 1-2 hours hitting a small park area. If I rode hard all day following a big snow dump there's no way I was going back the following day. Or if I went back is was on groomers hanging out with someone who's just learning. My legs were too cooked and I needed the recovery time. You need all your skills and muscle freshness to tackle terrain that's advanced for whatever level you're at, and showing up still trashed from yesterday isn't a safe move.
In short, I'd focus on more riding and trying to dial in your technique. Smoother carving and being able to handle tougher terrain with ease. Next, do the training steps above which just in general make you bigger/faster/stronger. And finally pick the days you want to send it, the days you want to work on park features, and the days you want to rest or chill. Spending a day working on your carves or riding switch is a day well spent but won't tire you out as much as really pushing the pedal to the floor.