What you want to do is to increase endurance and muscular fitness gradually so that the stresses of draft work will not cause excessive tiredness, sore muscles and/or pulled ligaments. Begin with short, easy workouts, and gradually increase to longer, more strenuous workouts as the "team's" fitness improves.
Some great conditioning exercises include walking, short jogs, retrieving and the ideal exercise—swimming. Remember to consider your environmental conditions as well. Temperature, terrain, ground condition, timing, and distance can make a difference to the safety of both you and your dog. Get out there and have FUN!