I am 37. I consider myself to be middle-aged.
Big fat hairy deal. It takes more time to lose weight and recover from exhaustion, and less time to get tired in the first place. That doesn't really put limits on what I can think, or feel, or even really on what I can do (though it might change the way I have to go about it).
Hey, 70-year-olds have hiked the Appalachian Trail. If I quit smoking and pull out of this death wish and inability to summon up the give-a-shit to get active and take care of myself, that could be me. I wouldn't be setting any land speed records while I did it, and I might want to pack extra arnica gel, shoes with really excellent support, and a frame upon which to elevate my sleeping bag (might even want to "cheat" with nights in hotel rooms when the Trail passes close to a town)...
...but lately I've been telling myself that that fantasy I gave up in my early 20s could just possibly become a goal.
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"Alas, our dried voices when we whisper together are quiet and meaningless, as wind in dry grass, or rats' feet over broken glass in our dry cellar." --TS Eliot, "The Hollow Men"