
Personally I would think your older boy could train with you, getting fitter as you do and getting used to doing more miles. I'm not sure there's any reason why he can't do the full marathon if it's built up to. I know some will be concerned about pavement pounding but it's walking not running so I think the wear and tear will be minimal in a young. fit dog. In a large breed your 9 month old probably hasn't stopped growing yet so you do need to be a lot more careful with him, 3 miles in 45 mins is quite a brisk pace, I'm not sure I'd want a large breed pup doing that.
My lot occasionally do 15 mile walks, over the hills for a pub lunch and back, they're off lead for most of it so probably doing double the mileage and they never have any ill effects afterwards. Before I lost my old collie girl she done the same walk, at 14, and wasn't even stiff the next day. I've got a new puppy now though, well she's 6 months so we won't do that walk again until she's over a year, probably 14 months or so.