I have walked Gunung Gede (2958 MASL) on Java up and down in a day, that is around 20 kms and 1700 meters up (and obviously down) in a day.
I have also walked from a bit east from the abra de Portachuelo (4700 MASL) beyond the Lagunas de Llanguanuco (3800 MASL) in central Peru, because we couldn't find a single vehicle to take us down to Yungay, until well after dark a truck finally passed by! That was around 35 kms with sideway birding walks included, at an average altitude of more than 4000 meters. Breathtaking landscapes included!
During lowland birding / flatland birding, I often do around 30.000 steps a day. This would be around 30km if hiking, but ofcourse, while birding, you sometimes take small steps or just move a bit on the spot.
I only do this when I have to, because you obviously walk past good birds during such walks...
But sometimes, you are better up the mountain at first light for e.g. parakeets (Santa Marta), Volcano Swiftlet (Gede), striped babblers (Philippines). And if there is no road going there, you start at 3-4 in the morning and at first light you often have walked 10kms and done something around 750 vertical meters. The last couple of years, more and more rental cars are involved in my birding, and sadly, it takes away some of the walking. But I try to do as much walking as possible (and necessary) because it's a good way to keep you fit.
I am already looking forward to walk towards some Striped Babblers, Sillem's Mountain-finch, Snow Mountain Robin,...