Knees
The following is how I think my knees work. I am not telling you how your knees work.
In the beginning
When I started running, one knee would always chirp at some distance. As time went on, that distance increased. Eventually it didn't chirp (although I did get orthotics, see below).
I fall a lot. Historically, that was due to either poor eyesight combined with wearing sunglasses when I shouldn't, or getting lost in thought. Over time I've taught myself to take off the sunglasses in dim light or even in bright light with tricky shadows. Although I've made some progress in not getting lost in thought, it still happens.
In general, when I fall and scrape up my knee, there might be minor discomfort in that knee for a day or two and the scab would be gone in two weeks, leaving behind a veneer of scar tissue at worst.
A dozen years later
On September 10th, 2022, however, I took a pretty nasty fall while running the Big Cottonwood Marathon. That scab didn't completely disappear for five weeks and the external scar tissue didn't go away for about six months. I had intermittent knee pain for about four months, but I kept training and it fully resolved.
Orthotics
When I was in eighth grade, I stress fractured a small bone inside my foot when I was jumping out of a swing for distance. My dad was in the Air Force, so I had free medical care. The foot doctor that I saw told me that I had extremely high arches and that I needed custom orthotics or I wouldn't be able to walk when I was thirty.
The custom orthotics were made of some extremely hard plastic. I was told that the plastic would never break, but to be very careful if it did, because it would be extremely sharp. Within a year or two, using a spade, I broke the orthotic, without cutting my foot. The pieces were indeed fairly sharp. I threw them away and never wore them, or any other orthotics, until after I took up running.
So, the claim that I wouldn't be able to walk when I was thirty was bogus. However, when I took up hiking, I was having some knee pain and a friend of mine introduced me to a woman who made custom orthotics. She made some for me and they seemed to help. Eventually, after having worn through a pair or two of her custom orthotics, I found she had retired, so I switched to another orthotic maker. I was his problem client. It took a bit of fiddling around to get them just right, but I've been wearing his orthotics ever since. He is now mostly retired, but still sees some of his old clients.
At one point I decided to forego the custom orthotics because I really didn't know if they were working or if it was more akin to all the ankle support I initially needed when I was hiking and in the beginning of my running. As my ankles got stronger, I needed less support. So, perhaps I didn't need these "fancy" orthotics after all. Without them (and it's been so long since I experimented, that I can't remember if I went with no orthotics or off-the-shelf ones), my knee discomfort returned.
From my point-of-view, it's possible that if I had continued to train without the custom orthotics that my knees would get used to it (or the nocebo effect would wear off) and my discomfort would go away. However, I also figure it's possible that I'd do damage and I already had the custom orthotics, so I went back to using them.
(Possibly incorrect) Conclusions
I do not think my knees are "consumable." I do not think that running— even with a pack—the way I do it, is damaging my knees. I believe that running strengthens my knees.
I do believe that falling damages my knees and I really am trying to fall less.
For what it's worth, I know many people who have injured their knees playing basketball or soccer and a few who have injured their knees running. I've torn a ligament in my left shoulder and I'm very glad that I run on my legs and not my arms.
I do not wish knee damage on anyone. I would hate for people to emulate my behavior and damage their knees. I am, however, quite glad that I've continued training even when one of my knees (typically, the one that I fell on most recently) "chirp".
I'll admit it: I don't know how my knees work, but I have my strong suspicions. I don't know if what I'm doing will eventually catch up with me, but I've been training hard for the Bataan Memorial Death March (or a substitute in the years when it wasn't held in person) for well over ten years. My training begins in mid November and lasts until mid March. During those months I'm running with my pack two or more times a week most weeks.
Sunglasses Addendum
In Silverton this year (2023), I bought a pair o sunglasses that were designed for trail runners and specifically address the contrast issue. Unlike some of my fancier sunglasses from the past (when I had ridiculous amounts of free cash), they are not photo-gray, but so far, I've found them living up to their hype. I wear them in situations where I wouldn't have worn my other pairs and I haven't—knock wood—fallen yet.
Unicycle Addendum
This deserves its own page.
I started riding a unicycle at the end of June (the 28th, to be precise) 2023 after effectively a 30 year hiatus. I'm trying to ramp up my mileage so I can ride a 100 mile event in Alaska at the end of March in 2024.
My knees were absolutely unprepared for this and although I didn't get knee discomfort while I was riding, There were times when I woke up in the middle of the night with fairly intense knee pain (in just one knee) that has lasted for about a half hour. Surprisingly, the pain always diminished to discomfort and was typically gone by the time I awoke.
This mystery pain happened a few days in a row, but never more than three days in a row, if I remember correctly. It was something I was paying attention to, and I became concerned, but not worried.
However, when I had the trouble I did not back down my mileage; I increased it and the problem went away. Eventually, however, I did back down my mileage, but that had to do with my right foot healing up and me wanting to do a lot more running to regain some the fitness I lost (and to lose some of the weight I gained).
So, AFAICT, unicycling put different stresses on my knees than running had and my body adapted to them. Yay.