I take my Boxer out often - the normal loop is about 10 miles with 1600" of climbing - no sweat, she does great. About 2 weeks ago I took her out on an 18 miler with nearly 4000" vertical - and she was slowing way down the last 4-5 miles. No leash - and she stays right with me. I try to only take her when i know there are going to be few people around - not because she will wander or chase, but she seems to make people nervous because of her looks.