Close to 2500m climbed up and down in one day and in 9h of hike (I ran in the end to escape a thunderstorm) in the Pyrenees, when I made it to the top of the Punta Alta starting from Pont de Rius in Val d'Aran. I mis-read the map scale, and moreover underestimated a bit the hike down to Estany Negro... I wansn't feeling tired yet while reaching the top, but I felt it was getting really an insane hike as I was hiking up again to Cap deth Port over the Restanca hut, while threatening grayish cumulus were growing as fast as mushrooms. Past the Cap deth Port, I ran down until the car scared as a cat, under thunderbolts...
It was in late 2000, some ten years ago... I was young
Since then I have hiked many times close to +/-2000, but I have the slight feeling that over this limit, an additional 50 or 100m more gets exponentially heavier to cope with, depending on what you have totalized before during the same day...
Getting older I have also the feeling this is not only a question of musculature and endurance, but also accustomization of ankles and knees to cope with the amount of steps. Only training on wild terrain real mountains, like having spent the several previous days in mountains, can provide a genuine training for that.