From another human is hard, but I've been on a couple of solo hikes. Overall, the feeling of isolation hits for me at frequency of human detection rather than distance, i.e. when I can see no human and nothing built with human hands for a while. Volcanic craters on rainy days are a good place since you can see all around you and it's wilderness so you can get a good idea that no one's in the crater with you and the jungle obscures the rest.

Especially now, with my girlfriend I think in the last year I have not been more than 100 m from the nearest human. Maybe riding my bike back from SF's business district, but those have security guards in them, so realistically 20 m.