I’m not a trainer or expert by any means, but I have a border collie-Australian Shepherd mix. I adopted him when he was 10 months, and have regularly walked him 3x/day. Usually each walk is from 20-45 or more minutes long. I wouldn’t be worried about long walks, unless you’re topping like 15 miles per day. Try adding chondroitin, glucosamine to his diet or buy food that has those added. They might help prevent hip dysplasia.