I have a 16 month old male and we do our brisk walk in a field that measures out to 1 mile and it takes us 15-20 min. depending on how many trees he desides to visit, LOL, does that give you some idea of how fast he trots
Then we do a 1 1/2 mi. off lead walk in the evening 3-4 times a week at our breeders and that takes him 20 min. and me alittle longer sometimes :D. All total in a days time he gets anywhere from 2 - 2 1/2 mi. at a pretty good trot. Don't worry it's a safe place to walk off lead, our breeders house with 50 + acres. An hour or longer of just walking could be a bit much and I never walk on hard surfaces, too hard on the joints, at least till their plates close.
Judy