The following formula provides a more accurate heart rate measurement.  Instead of calculating MHR, use your resting heart rate and your age as factors in calculating PHR.  The formula follows:

For each type of workout previously described, there exists an associated percentage of MHR, or exertion effort, at which to train.  For example, if a 34 year-old walker maintains a resting heart rate of 50, then estimating proper training heart rates look like the following:

In contrast, using the traditional HR calculation method produces lower heart rates.  If you are untrained and just beginning a race walking program, these approximations are probably acceptable initially.