My father introduced me to hiking. I usually didn't do it with a pack, but with a rifle during hunting season. The distances we covered weren't huge, maybe three to five miles in a day, but to my ten year old self it seemed like I'd been through a true ordeal. Fast forward to the military.
My father's hikes were designed to keep me from getting too tired. He wanted me to have a nice time. Every hike I encountered in the military seemed designed to make my life as miserable as possible. I had Inadequate sleep, because we had to be up at 3:45 to get our weapons. I had inadequate stores of energy in my body, cause I hadn't eaten breakfast. My pack was too heavy and the twenty pound rocket launcher and eight pound rifle I had to carry didn't help. Neither did the flack jacket or kevlar. Moving the gear around didn't help. One thing you learned was that you were going to be in pain. There was no way around it. No matter how you arranged your gear, or carried your weapons, it was going to hurt. In the words of Stephen Pressfield, the Marine Corps taught me to be miserable.
That might not seem like a valuable skill, but it has served me well. I find that there is a separation between my body's pain, and my mind. When I am in pain, I no longer need to respond as if it were bad. This has helped me immensely during my conditioning training. It has allowed me to set aside my pain, and figure out a way to lessen it.
When I ride my bike, if I am pushing hard, my legs burn. If I switch my breathing pattern so that I am taking in more air, the burn begins to subside. If I'm doing burpies, instead of concentrating on my pain, I concentrate on economy of movement.
If there is one thing I have learned about fighting, it's that you are going to be in pain. No matter what you do, you're going to get tired. You can't escape it, if you try you're probably going to get hurt, or put on a weak show. The best thing you can do is accept it. Live there, in the pain, and remove your attachment to it. This was taught to me by hiking.
If you are reading this and you feel you need to increase your mental toughness, strap a pack to your back (35 pounds will do) with a little food and gallon of water and go walk twenty or thirty miles. It doesn't need to be in the wilderness, it could be around your town. I suggest you walk for three hours, then take a one hour break, rinse, repeat. Take some mole skin, wear some moisture wicking socks. If you've never done it before, you'll want to quit after the first hour. You'll probably tell yourself that it was a stupid idea, or that it isn't even a good way to train. You'll come up with many reasons to quit. I suggest that you don't.