It's not that walks are not good, they are. They're not enough to tire a dog out. So you can walk 10 miles (not with a pup) as long as you then still have it in you to do some exercise, as in your dog running.
I can't run and it's hard to find a good place to let him off leash, so I'm always struggling to find a way to exercise him. Fetch, tug, chase stuff, as long as he gets to run.