I had my entire large intestine removed 14 years ago, so I have an ileostomy. My stoma was placed nice and low at my request, and as a result it doesn’t interfere with a climbing harness or backpack hip belt. During those 14 years, I’ve done martial arts, running, climbing, crawling around in caves, rappelling and ascending ropes in caves/cliffs, and obviously backpacking. My ostomy doesn’t slow me down at all. A major function of the colon is to reabsorb water. I lost ALL of that function and consequently poop nothing but liquid. You still may have the majority of that function intact since you have a portion of your colon remaining, although I’m not sure how much you still have. If your stool is fairly formed, then you’re not losing near as much in the way of fluid and won’t require as much diligence in staying hydrated.
For backpacking you’ll need to have already figured out how you will change your bag/flange since the bathroom sink, countertop, etc won’t be there, and you'll have to determine what supplies to take along. I’ve yet to have to change the flange on a trip, but I’ve had to change the bag a couple times. You’ll have to figure out over time how frequently you typically need to change these, because it varies widely between individuals, and this will factor into how much supplies to take on a trip. I can sometimes go a week or more before needing to change mine, but most can’t do that. As mentioned previously, the hip belt placement may be an issue. If you solve that riddle, I’d recommend doing some shorter day hikes with a fully-loaded pack before venturing out too far, because you need to figure out if your hip belt is doing anything to compromise the adhesive on your flange. It does NOT on mine, but then again, my stoma placement is probably lower than yours.
It’s normal to have a gazillion questions post-op, so feel free to PM me ;)