Me and the old feller run collie greys on rabbits, nothing else, during the day. The dogs need stamina for lots of runs, and a turn of speed to nail them before the runs get too hairy! (Rock edges, and bomb holes).
It takes us 3-4 weekends at the start of the season before the dogs are properly in gear, (Speed and stamina). At this point, the food intake is greatly increased to feed the metabolism of the working animal out twice a week.
I've seen dogs in the past go out nearly every day, and perform strongly when fit. (but need lots of extra food, and lack that dynamite edge, but only just,