at that age could be HD and/or arthritis, as bird says best thing to do is get as much weight off it as you can without it being unhealthy. And keep walks short and relatively frequent rather than longer walks.
If you do go to the vets and they give you metacam or previcox for arthritis, insist on regular blood tests and don't give it to it daily (as they sometimes recommend you do). Are you from Wrexham? I'm also from Wrexham and took my mastiff to Wrexham Vetinary Care with his arthritis, they put him on previcox and advised me to give it to him daily for the rest of his life. They n