I know its difficult with mature dogs.They dont really want to play or gallop,and a lot wont chase balls and things. A gallop with you whistling him whilst someone holds him is useful, on some grass or field margain.I do this or like you have already done a steady canter behind the vehicle for 3,4 0r 500yds or let him run round in woodland.The smells usually get them going,but they can get into trouble or taken off by roe scent etc. I use a greyhound walker aswell,especially before the season to tighten toes and things up.It also encourages steady breathing.Start with 10mins once hes used to i