For my part, if the terrain is undulating, flat, in green footpath areas, like Kent and Sussex, for example, I can still easily hike between 10 15 miles a day, if I push it I have been known to even get up to 20 miles in a day. It all depends I suppose as to what I'm motivated to do and why (e.g. is there a pub waiting for me to visit, etc?) what the weather is like at the time and so on. In the Lakes or similar, I would walk less distance per day unless I desperately wanted to end up in a pub at a reasonable hour. I find pubs very motivational!