I've spent a lot of time in some pretty deep wilderness in the west, and I'm skeptical that I've ever been more than 10km from the nearest other person. I often come across old tracks even when I'm going cross-country. Even some of my secret spots are now getting a lot of traffic (see also https://www.youtube.com/watch?v=37Hmd-VkMIM). I climb mountains just to get some peace and quiet and half the time there's some other unfortunate soul trying to do the same. I think my most remote trip so far was somewhere in the vicinity of 10T 480360E 4595500N, and even then, I doubt there was more than 10km between me and the next nearest person.

I think what most people are thinking of when they think of wilderness solitude is density of people. The next nearest person might be only 5km, but depending on conditions, I'm not likely to realize they exist. At densities of < 1 person/sq. km., most people start getting that feeling of being "really far out there".