When selecting an initial goal, start with shorter distance races first and work your way up to longer races. The training program for each race goal includes a minimum distance to walk the first week. If you typically walk much less than that distance, you need to build up to the starting distance slowly. Add a few miles each week until you bridge the gap. Then assess your easy walking pace and try to match it to a program. This provides a fairly accurate method for determining a realistic goal.
Once you start a training program, reassess its appropriateness after about three weeks. If you feel you cannot meet most workout target times, switch to a slower schedule. Use common sense and do not train through injuries. Do not stay on a schedule if you constantly feel sore or tired, like you completed a marathon every day.
I designed each program to train walkers at the minimum sustained pace and distance needed to achieve a targeted goal. Some people need more training, others less. My programs offer a safe approximation of the work you need to accomplish to ensure reaching your goals without getting hurt or over-training.