Actually, this subject is really important.
It is not so much our weight, but our overall fitness. Here is a little bio about me. Please don't take the comments wrong. I am only a guy who likes to hike in solitude, and have been doing it for a long time. Just want to share some simple things I do, and perhaps it might help others.
I am 58 and weigh about 5 lbs more than when I was 18. I am blessed with a fairly high metabilism. But I do stay active year round. Although I have moved to UL, there are still times I need to carry a fairly heavy pack (usually a lot of water in the desert), and it is not as hard as one would think. I can still pack 40 lbs fairly easily... not that I really want to... well other than sore hips, fairly easy!
Last month I went on a week-long backpacking trip with my son, who is in college and is a cross country/middle distance runner. He runs about 90 miles a week. Actually he is pretty good. Last year he ran a 1500m in 3:46, which converts to about a 4:04 mile.
Anyway, I had not gone packpacking with him since he was about 10 years old. On this trip he was absolutely shocked that I could keep up with him. I was not trying to impress him, we were just moving along at a reasonalbe pace each day.
I am not bragging. I don't excercise a lot. I usually go to the gym maybe 3 times a week max, and exercise for less than an hour. Sometimes I don't go to the gym for a couple weeks at a time. Currently, it has been over a month since I want to the gym. I take my dog on a long walk two times a day. I do run off and on, but nothing serious... work alsways seems to interfere with getting into any kind of running shape. On weekends, I usually hike with my wife or go backpacking by myself. We live at 400 feet above sea level, but are close to the PCT in the San Jacintos. She has never backpacked, but we both have no trouble hiking this section of the PCT which gets up to about 7,000ft, which is maybe a 2000+ foot elevation gain from the trail head. We often do a loop that is about 16 miles, only carrying day packs with water and a few items. I don't have any problem averaging 15+ mile days on my packpacking rips carrying up to 20lbs or so. I guess the important thing is that some sort of excercise/outdoor activity has just been something I have done all my adult life, because I enjoy it. Once you get into shape, it is not that difficult to stay in shape.
I don't necessarily watch what I eat, just don't eat too much. We are not "into" fitness, we just take a little time every week to enjoy ourselves outdoors.