I agree with WD. I like to keep my dogs active all day. It keeps them in shape and sharp in the mind. In the summer, you can't keep them out of the water! Swimming seems to tire them out more quickly. :) It's the resistance in the water. Throw the tennis balls in the water to retrieve and what a great workout.
I don't think a stroll is going to do any joint damage. He's already 12 months and could actually handle a little more strenuous excersise safely as long as it wasn't excessive. My little guy is going to be 11mos Feb 10th and he does everything the bigs dogs do with no problems. I just keep a close eye on him and if he looks like he's laggin then I make him take a time out or take him for a drink and let him rest for 15 mintues or so. He should be fine on your strolls. If he looks tired, rest and offer some water. Continue when you think he's ready.