Interval training is a form of exercise where the pace of the training is purposely speeded up and slowed down at regular intervals.
Burst of fast-paced walking can gradually be introduced to your walking routine. Walk as fast as you can at a pace you can maintain for a short while only, then slow down to your normal walking pace again. As you get fitter, it would be possible to increase the intervals of fast-paced walking until you can walk for 50% of the time at the higher intensity.
Use a wrist watch with a minute hand to time the intervals, for example two minutes fast, two minutes slow. Another technique is to use street lamp posts or similar landmarks to time the bouts of fast and slower walking.