It's partly genes, of course. Like with anything, look around and everyone you know has different abilities according to their individual gifts. But humans are designed for the outdoors and for hard physical endurance. That's in all our genes. Most of us have just not been out there enough and trained hard enough to allow our bodies to develop to that level.
In 1996 my wife and I set out on a 6 month bicycle journey around Europe. We camped every day and spent almost the entire six months out under the sky. When we started we were in bad shape, barely able to get through the first days without giving up early in the afternoons, still far from our daily mileage goals. But as the days and weeks and months passed we found that our bodies hardened and developed resistance to temperatures that we never imagined we'd be able to reach. At the end of six months when we returned to Japan in November and stayed with my wife's parents, we found that we couldn't stay indoors without the windows open, even though it was very cold outside. Our bodies were stronger than they'd ever been before or since.
Bill Merchant has been living outdoor most of his life and had a lot of time to develop both his body and mind, much more than most of us. This alone ensured that he would develop the strength, stamina, and endurance that he has, plus the knowledge and skills necessary to know how to live in the environments he so excels in.
Also, you have to take such articles with a grain of salt. Any time you read words such as "master of the Alaskan wilderness", "I was spellbound", or "coming into his full glory" you have to stop and shake yourself awake. Bill Merchant is just a man, not a god or super hero. The flavor of the article affects the image of the subject as much as the truth of the subject himself, so you are not getting the entire picture when you read such articles. I was the one who recommended the article for the "Classics of BPL" thread the other day, and I still think it is a great article about an amazing man, but it is also important to remember that what he has done is not humanly impossible.
You are an athlete, Huzefa. Just get out there and start training yourself. Acquire the knowledge you need by actually getting out in the field rather than just reading books. You will learn much more and gain the strength you need by being out there. You will also learn your limits, which you can't learn from sitting in front of a computer. You are much stronger than you realize, I think. Get out on the slopes and in the rain and along the trails that go on longer than you thought and keep walking. Your body will refine itself to all that.
I am 47 and for the past few years allowed myself to get out of shape and I also allowed my diabetes to dictate my lifestyle. I began to lose confidence that I could be strong again. Then in January I started training again (with Crossfit ) and have started gaining back a lot of my muscle tone and strength. Yesterday I did 53 pull ups, which I never thought I'd be able to do again. I am running again and this summer I hope to get back up to the more difficult peaks that I'd given up on. If I can do it, so can you! But you have to keep at it and believe in it!