When I was 27, I joined Hikanation (precursor to ADT). I hiked 3300 miles, carrying a much heavier pack than is typical today, usually in the 50 to 65 pound range depending on where we were and time of year. I hiked 11 months and had no problems other than a cyst which developed on my lower back from the hip belt rubbing - this eventually went away, and the typical blisters due to the heavy leather boots that were the rule back then and the heavy packs. I had no lasting physical problems from the hike.
I’ve continued hiking several times a year since then, but nothing that extensive. I am 53 now. Last summer I did a 170 mile section hike with no problems other than a numb toe - I think this is a result of the much lighter weight hiking shoes, MUCH lighter packs, and pushing substantially more miles than we used to do. The PA rocks and flooding may have had something to do with it too!
I’m sure everyone’s mileage will vary in this regard, but I don’t feel that I’ve done any permanent damage to my body. I did switch out to lightweight gear and use trecking poles now to help assure no problems develop now that I’m getting on into the second half century.
Lyle