If your race goal is to place in the top three or beat a certain person, then strategy becomes a very personal plan. No strategy--ok, aside from cheating--enables you to beat a walker who typically walks ten minutes faster than you. However, knowing your competitor as well as your own limitations gives you an advantage over a realistically chosen rival. If your competitor makes a habit of starting too quickly and then slowing down, let your competitor take a lead at the beginning. Keep him/her in sight, walking within your own limits. Then, as the race progresses, catch up and pass decisively. Do not just catch up to a rival and walk together. When you make your move, it must be checkmate, with no response. Also, never be tempted to look back.
Some walkers like to suck off your pace. This creates a very frustrating situation. If your competition tries this, you have two options. Speed up and try to lose them, or slow down and make them take their fair share of the lead. Often, two walkers racing together agree to switch the lead on each lap, and then race for the victory with a few laps to go. Keep in mind, if you are racing for time and not place, keep walking at your pace. Use these strategies only when you race for a place.
Regardless of your race strategy, always make life easy on yourself. Walk behind or in front of your fellow competitors. Walking alongside them, unless they are on the outside of the course, is equivalent to walking in the second lane of a track; on a course as curvy as a track it costs you 8 meters every 400 meters. In a 20K race, that means a whopping 400 extra meters.