When I think about walking my dog, I've found that there are three different types of walks.
This is a walk that belongs to the dog. It is all about sniffing, exploring, observing, scanning, lingering, all things dog. All dogs need to have opportunities to explore the world around them and what better way than on a walk. This walk is totally directed by the dog, I'm following his lead.
This walk is for the human. I gave up my gym membership many years ago so that I would have the time and energy to walk my dogs everyday. I love walking and typically get 4-6 miles in per day. When I head out on my walk it's pretty faced paced so I typically take my older dogs who are seasoned walking buddies and can keep up with me. It's important to note that 'forced exercise' is not appropriate for puppies, senior dogs or dogs without appropriate endurance.
These walks are geared towards training and socialization. When I have a young dog in training we spend alot of time walking and training in public. These walks include working on leash manners, heeling, greeting people and ignoring distractions. I'm not concerned about the distance we walk and often times we are just going places and hanging out, so this is not meant to be a physical workout for either human or canine.
By determing which walk you're going on you can decide if you should include friends or just walk alone. I've found over the years that both myself and my dogs get more exercise and enjoy our walks when we abandon the one-size-fits-all approach to walks.
More great info by fellow trainers.