Oh! Sorry I had thought you had said the opposite of that.
For "great walkers, very fit", I'd expect 3.5 to 4 mph, especially if the walk is as short as 1 hour. I.e. my regular lunch time walk is 3.3 miles, which I do in 55 minutes, i.e. 3.5 mph.
For a charity walk for "great walkers, very fit"...