The trail was bad, and I felt half mad, but I swore I would not give in; (line 39)
This is pretty much his flat-out statement of perseverance. The road’s lousy, he’s going nuts, but the last thing he’s going to do is give up. Our guy is definitely a survivor, and it’s that will to keep going, that focus on the goal, that helps him stay sane and alive on the Arctic trail.
I do not know how long in the snow I wrestled with grisly fear; (line 53)
Even a tough guy has some dark moments. He survived the trail, but the actual act, the cremation of Sam McGee, is a little bit tougher for him. He has to really struggle here to get through it, to conquer his "grisly fear." That’s definitely part of the meaning of perseverance in this poem. Not just being able to stand a little cold, but being able to fight your fear and win.