Just keep them busy, 3 walks a day with mine - I say walks but with a Lurcher every walks hunting or finding something to hunt :-) tweak it up abit come the end of summer/autumn with some short sessions lamping. The key is variation, running, trotting, walking, swimming whatever just get em out and about and feed them well.
I can't see how you can be fat and unfit with a Lurcher about unless you run them behind a car or put them on a walker lol