Treadmill isn't the same as real walking and flat land work doesn't compare to mountains. Swimming doesn't carry over very well, neither does bike work, tried it all and had numerous conversations with a guy that tried it all also and he had come to the same conclusions and he had been killing elk on the north fork of the St Joe for almost 30 years.
Sled work not only gets you over in nearly the same angle but works the same muscles as climbing a mountain, the only thing it leaves out is thinner air and the downhill.
Hill sprints are king above all else, but brutal in terms of will power to keep going and keep doing them. They are also amazingly easy on the joints.
A couple more I forgot about is weighted carries, I'm not talking about 50 lbs in each hand, 100+ in each hand is where the magic starts to happen. You won't believe how light you feel after carrying 300 lbs for 50 yards.
The other is single leg squats and it's progressions. The balance control, mobility and flexibility required are very good indicators of muscle imbalances.
Sprints are really hard on the body, you have go easy on them and make sure you are fully recovered before you do your next session, DON'T go on a schedule, go when your body is 100% ready. Sprint until your speed starts to fade and your wind doesn't come back as quickly as the first couple runs. This may be humbling the first couple of times, but leave the ego at home.
So it varies, it can be 3 - 50 yard sprints or 10 - 100 yards before you start to feel yourself get a little winded. I'm not talking about total exhaustion pile of goo, I mean when your speed is fading sooner and it takes longer to get your wind back. Go slow, go easy and work your way up.
Take your time, I cannot stress that enough. Getting stronger, faster and having more endurance don't mean jack squat if you are constantly injured.
I learned this lesson the very very hard way and deeply regret pushing myself WAY beyond what I should have, just because I was on a schedule and hunting season was just two weeks away. I ended up with tendonitis on my left ankle and knee that was so painful that even stepping off the front porch was nearly impossible.