When i was Mountaineering & Climbing i would train each morning when I was not actually Backpacking in the Smokies. I had a top notch pack that I would put 70 lbs in and made a drag with two truck tires attached with a rope to my pack. Would go up a lot of hills for at least two hours. every day. Had to be in top physical condition. When actually climbing a mountain we would carry 70-80 lbs in backpack and 40 lbs in a sled attached to the pack. You had to carry everything to survive for several weeks on your back and sled.
When I worked for a national Insurance Company I had the opportunity to do a Risk Assessment at a Manufacture that made all the Web Gear and Packs for the SEALS. I had to get a Government Clearence to go onsite. The owner of the company gave me a tour of the facility and equipment they made. Kevlar in just about everything with lightweight materials. The quality of everything was the BEST and Tripple QC before it left the facility. They made so many different configurations that all fit together. The cost $$$s to Tax payers was beyond my belief. The owner knew that I was into climbing mountains, backpacking and hunting and offer me some of the GEAR. I was stupid and didn't take the offer due to company police of not accepting gifts or products from potential customers. "Conflict of Interest" I should have disregarded it and got some gear. They made to coolest and usable gear. The SEALs and Special Operators get the BEST equipment. I still kick myself in the butt for not getting some of that gear for hunting.
All those years of pounding my body kept me in GREAT shape, but can't last forever. I had four knee scopes, Two ACL replacements, a total knee replacement, three shoulder rotor cuff repairs and one total shoulder, four spinal cord operations with five disc removals and C-2 to C-7 all Titanium, countless feet/toe issues.
I suggest keeping in shape, but not to the extreme.