You alternate a fixed run interval with a fixed walk interval from the start of the race, rather than walking only once you are tired. Jeff Galloway, the 1972 US Olympian who popularised the approach, built it around taking the walk breaks early and consistently. Common ratios run from 30 seconds of running and 30 seconds of walking for beginners up to 8 minutes running and 1 minute walking for experienced runners.
Work out how far you cover in one full cycle, then divide the cycle time by that distance. A 4 minute run at 5:30 per kilometre covers about 727 metres, a 1 minute walk at 9:00 per kilometre covers about 111 metres, so the cycle covers 838 metres in 5 minutes, a blended pace of about 5:58 per kilometre. This calculator does that arithmetic for any combination you enter.
Over a single kilometre, yes. Over a marathon it often does not, because the comparison that matters is your blended run-walk pace against the pace you can actually hold continuously for 42 kilometres, including the fade after 30km. Runners who fade hard in the last 10km frequently finish faster on a disciplined run-walk plan than on a continuous one, which is the whole argument for the method.
The one you can repeat for the whole race. Longer run intervals suit runners with more continuous running in their training, shorter ones suit first timers and anyone managing injury risk. Test the ratio on long runs rather than debuting it on race day, and remember that the ratio you can hold at kilometre 5 is not automatically the one you can hold at kilometre 35.
Every Abbott World Marathon Major has a course time limit rather than a rule against walking, so a run-walk plan is fine as long as your blended pace finishes inside that limit. Check the finish time this calculator gives you against the limit for your race before committing to a ratio.