Try adding walking lunges throughout your walk. Keep your dog by your side every time you lunge by pulling up on his leash, to signal that he should sit. Hold the lunge for as long as it takes him to sit down.
Add interval training to your walk, by running a bit for two minute stretches every once in a while. You can work your way up to five minute jogs – mix it up by adding zig-zag motions as you go.
Some well-trained dogs can even join you in fitness class, like run club, or you can join a pet-friendly exercise group that includes sit-ups for you and obstacle training for your dog.