We live in the city and have a 6000 square foot lot, which is pretty big for the city, but not nearly big enough to exercise 3 very active dogs. Luckily less than 1 mile away is a school yard that allows dogs when school is over/out for the year. If I'm home, I would ideally take the dogs to Marymoor dog park everyday. It's massive and beautiful and there is even a river for swimming. The only downside is that it's at least a 30 minute drive across the traffic laden 520 bridge. So...instead I often walk (ideal) or drive them to the schoolyard.
I spend the first 20 minutes walking laps around the field while they run around. They all enjoy running in no particular direction just for the fun of it. It's awesome to watch. During that I work on recalls. After this, I usually throw the chuck-it and work on "stay", pretty much until they clearly can't go anymore. To cool them down, I walk around a bit more until they aren't panting heavily.
We do this routine everyday, other than the day before a tournament and the day after. I think it makes a huge difference in my dogs performance and prevention of injuries.
I know it's hard to get them out everyday for many people, but it's so important if you have an active sport dog. I believe that it's really not fair to ask your dog to run the equivalent of a marathon with no physical training.
You can even teach your dogs to run on a treadmill while you watch TV!
Here are some clips of a typical exercise session