Irony I am 76 and had similar thoughts. When I went from DSLR in 2022 to Z9 the last f mount lenses I sold off were a Sigma 60-600 sport, a 500mm pf and my 600 f/4E so fairly heavy lenses that I had held for my on the go in all sorts of terrain and habitat birding bird ID photography addiction.This has been an interesting thread to me as a peek ahead. I’m in my early 50s so I still have some resemblance to vitality but have been pondering things like whether I should spring for a 600 f4 while I can (so to speak). Of course I have the 100-400 and 600pf so that may keep me from justifying a second longer telephoto.
My first Z birding lens was a Z100-400 with 1.4TC and on 5-1-22 a Z800 f/6.3. Later I added a Z600 f/6.3 (pf) and a Tamron z mount 150-400 to my birding arsenal. Then Nikon put the Z600 f/4 TC on sale my dream birding lens. I decided to sell the Z800 pf and some other unused z lenses and take the plunge on the Z600 f/4 TC while I still can as you so aptly put it I do high intensity interval training, core strenght founer exercises and weight work with free weights 3 days a week for a reason and have been since I was about 62. I was far less than "fit" at 50 so it is easy to say I am more fit now than I was at 50.
One never knows how long this will last and someday the tripods, monopods, and various other photography support gear in my closet from the pre VR lens and pre me being fit days will get used again So enjoy it while you can.