It sounds backwards: deliberately walking during a race should slow you down, not speed you up. Jeff Galloway, a 1972 Olympic distance runner, built an entire coaching method around the opposite claim — that planned walk breaks, taken before you're forced to, produce faster finish times than continuous running for a large share of runners, especially at marathon distance and beyond.
The mechanism
Continuous running accumulates muscular fatigue linearly until it doesn't — then it falls off a cliff, and your pace collapses in the final miles. A run-walk pattern (common ratios: 4 minutes running to 1 minute walking, or 1 minute running to 1 minute walking for slower runners) resets some of that fatigue at regular intervals, keeping the collapse from happening at all. You give up a little speed in the running segments' average pace and get back more in the segments you'd otherwise be shuffling through at the end.
This isn't universal — a well-trained runner racing a 10K is rarely better served by walking. The math tends to favor run-walk most clearly at marathon distance and beyond, and for runners without a large aerobic base.
Running the numbers
The Run/Walk Calculator takes your run pace, walk pace, and interval lengths and returns your blended overall pace — the number that actually determines your finish time, which is easy to misjudge by feel alone.
Worked example: Running at 9:30/mile and walking at 15:00/mile, using 4-minutes-run/1-minute-walk intervals, produces a blended pace of roughly 10:24/mile — about 54 seconds per mile slower than continuous 9:30 pace, on paper. The bet Galloway's method makes is that you can hold 9:30 for the running segments across 26.2 miles with walk breaks, when you couldn't hold it continuously past mile 18 without one.
Choosing a ratio
Faster, more experienced runners tend toward longer run segments (4:1 or 9:1). Newer runners or those targeting a first marathon finish often do better starting closer to 1:1 or 2:1 and adjusting up in later long runs. The only way to know your ratio is to test it in training, at goal race effort, over a distance close to race length — not to guess on race day.
The walk breaks work because they're planned, not because you're improvising them when you're already in trouble. Decide the ratio weeks out, rehearse it, and let the calculator tell you what pace that actually nets out to.