This depends upon a lot of individual factors,
especially the pace of the walk. In numerous surveys, we've found that most
runners lose 10-20 seconds when they take a one minute walk break. Let's say
that a runner has a goal pace of 10 minutes per mile. The standard run-walk-run
strategy at this pace is (run 3 minutes/walk 1 minute).

Most of the runners I've monitored at 10 min/mi
pace tend to lose about 15 seconds when they walk for one minute. This means that
for every minute run, the runner only has to run 5 seconds faster. This has been
very easy to do for the runners that I've worked with, when the pace is right,
for the person, on that day.