After doing 2000-mile treks on the AT and PCT, I can say that hikers generally get their "trail legs" after roughly 500 miles or one month. Damascus VA and Tehachapi CA are big spots for zeroes and recalibration, and then you're off to the races. This post is excellent in showing the changes in those first 500 miles, but doesn't go on to show much about what happens after. That peak performance is maintained very well.
I agree with your assessment generally, but there are some caveats.
When you start to get into around the 10th week and beyond, there's a wall you can hit if you aren't eating well enough. This is not really an issue on American or European long-distance trails where any sort of nutrition one desires can be arranged in your resupplies (and you can eat ravenously on town days) but if you do multi-month trips in the 3rd world, you'll find trail legs don't last forever when all you're eating is rice and some kind of legume for every meal. At a certain point the lack of high-quality nutrition starts to make you progressively weaker.
That was exactly the case on my 18 month bike trip in Alaska, South America, SE Asia & China. We found it very difficult to pack or even find enough quality proteins (and at the time we didn't know they where that important tbh). In the end I lost 20 kgs and went from 85 to 65 for a 178 cm 30 y.o. guy.
That was 5 years ago and I managed to mantain that weight by taking care of what I eat. I think that trip changed something in my metabolism and helped to know myself better.
