I lived on a mountain in the jungle for 18 months. I can testify from personal experience and observation of others that your endurance increases as time goes on, even increasing after a person enters old age. I lived among and worked with a group of native tribal people, and the 60-70 year old mountain men (those who have remained active and mobile all the time, not laid up with injuries or illness) could easily out-pace the younger people on the trails. The younger people could run faster and longer, but the old men (and women) could walk a good 3 times faster than the younger people, and could do so for much longer before getting tired. It sounds strange that those who can run faster and longer cannot also walk faster and longer, but it was such an obvious fact that it was common knowledge among those people.

Similarly, a seasoned adventurer would deal with the rigors and hardships of the adventurous life much more easily than a younger, less experienced adventurer.