I agree, let them run.  Im lucky in that if I am short of time or weather restricted, I have a pond that I use to get a good dose of swimming in on the short days.  The most common error seen is letting a dog get fat.  Even if you cannot exercise daily there is no excuse to let the dog get fat.  This only complicates the dog getting in shape later when the season starts.

I heard that having the dog swim in the water or water retrives work good for stamina. But also going out and walking all over with the dog as well, like everyone said.

There are two things you can do to condition your dog. Over winter we do this with all the dogs we have to prepare them for training and trialling. Or a dog that comes in for training that is over weight and out of shape. This is a strength and endures program. Part one - the strength building- buy a padded roading harness, put 5 ft of chain on both sides and take the dog for a walk, start out on very short walks, and build up slowly anding more chains so the harnes. Our dogs end up with 35 ponds at the end. This is done over weeks, Part two endures - We us a treadmill to run the dogs on, fist start out with the dog on a trot, for about 15 min per day, building up the time spent on the treadmill every three days. We do this until the dog is up to 45 min.  then you can add inclines to the treadmill.