I try darn hard to get my dog out for a walk at least twice a day. By walk I don't mean potty breaks. I mean real "walkies" at least a mile, usually longer, but a mile is the minimum each walk. In the evening we try to get in an hour or as close to it as we can.
Of course if we have our Agility lesson or are doing other training that night we may shorten the walk etc. its not a hard and fast rule.
In the Spring, Summer and Fall we try to do some running preferably in the morning when it is cool and then do our "walk" in the evening.
What I have found is that a 3 mile walk will tire Reese out and satisfy her exercise needs more than a 2 mile run will. Granted she is running on leash so it is a run for me and a trot for her.
But....the walk incorporates times to sniff, poke around, it isn't done as a race. It is a relaxing time for both of us to unwind and spend quality time together. When we run it is more business like. Minimum sniffs and potty breaks, and preferably we get the poos taken care of prior to the run.
So to be fair if we run in the AM it is business, but our walk is all about the relaxation and sniffy's. I allow Reese to sniff as much as she likes within reason. I still have her walk with me a bit each walk, meaning next to me on a loose leash, but not technically heeling. Then for her reward she gets to "go sniff". The rest of the walk she can walk in front of me stopping to sniff and mark as long as the leash is loose.
Some people believe that anytime the dog is in front it is in control and being "dominate" but I don't believe that. I don't like pulling on the leash, but if my dog is being respectful of me or walking on a loose leash and is aware of where I am, so that if I slow down she slows down or if I stop she will stop and wait for me, I have no problem with that.
I believe that on our walks Reese is getting mental exercise by sniffing and exploring, using her nose and having more time to take in the environment, plus she is getting physical exercise by walking. A walk actually meets more of her needs than a run does.
If we run three miles Reese will be physically tired and sleep the rest of the night. However, if we do less than that, say two miles, Reese is ready to rock when we get home. Two miles is just enough for a warm up for her. I think it actually gets her more hyped up but doesn't satisfy her enough relax her. After a 2 miler we need to do some trick work or other work that will work her mind. This is what Patricia McConnell was talking about on this blog post http://www.theotherendoftheleash.com/just-enough-exercise-to-wake-up-our-dogs . I think she was right on.
I think each dog is different and you have to find what works for your dog. Reese has an off-switch which is wonderful. But I also recognize that she waits for me while I am at work all day with not much to do but look out the window and sleep. I owe it to her to give her both the mental and physical exercise she needs to be healthy. For Reese, as Patricia said in her post, a 2 mile run "wakes her up". A 3 mile run is enough to tire her out. That being said given the choice of a 1 mile walk or a 3 mile run, the walk would win. Why, because it involves both the mental and the physical aspect for Reese.
In her book Animals Make Us Human Temple Grandin talks about animals emotions, one of which is Seeking. Seeking is the animals need to search, and investigate its surroundings. Dogs do this on walks sniffing and exploring the environment. Which is another reason I try to vary where we walk. It is more stimulating for Reese if she can explore new places, and who has been there before her:)).
Finally, I believe that walking with your dog is such in easy way to spend time bonding with them. Walks are highly reinforcing for dogs and spending time walking with them will reinforce your importance to them - they don't get to go without you after all. It is such an easy thing for you do to together.
I love what walks have done for my relationship with my dog, and I always feel better after I get off the couch and hit the road!