They need exercise - but not necessarily a walk. Play taps in to more of a dogs natural behaviors. If you walk your dogs in the morning, play a game in the afternoon - play fetch or tug. You can set up little agility courses, learn ball herding or learn nose work to mentally stimulate as well as exercise your dog.
It all depends what breed of dog you have. Some need lots of exercise, while others only need a short stroll. You should walk your dog twice a day, and then the length of the walk depends on what kind of dog you have.Remember, walks are not just for exercise, but also the chance for them to relieve themselves.
I own a great dane and a dalmatian and they definitely need different amounts of exercise. For the dane a 10-15 minute moderately paced walk is enough for her. She's really just about getting out the house more than for exercise. The dalmatian on the other hand needs the exercise. He is a high energy breed that was originally bred for running behind horse carriages and guarding them at night. He could easily run 20km a day however an hour a day and he's a very happy man. If you manage your dogs diet properly so they are not overweight and make sure that they get cardio for heart health and mental stimulation from toys or play and interaction you are doing better than most dog owners. The amount of exercise required definitely depends on the breed over dog size. Breeds like fox terriers are likely to need a lot more exercise than say a newfoundland just because they are a high energy vs a low energy breed. Personality can have a lot to do with it too. No one size fits all I'm afraid!