The workout is 10 repeats of 800 metres with a recovery jog of roughly the same duration between them. The rule of thumb from Bart Yasso is that the minutes and seconds of your average 800 time translate to the hours and minutes of your marathon finish. Average 3 minutes 30 seconds per 800 and the rule predicts a 3 hour 30 minute marathon.
They are a checkpoint, not a prediction engine. 800m repeats are driven mainly by VO2 max and lactate tolerance, while a marathon is driven by threshold, running economy and fuelling over three to five hours. Runners with strong speed and low marathon-specific mileage tend to beat their Yasso number in the workout and miss it on race day. Treat the output as evidence about one system, alongside your long runs and tempo work.
Whatever pace holds for all 10 repeats at even effort, which is the point of the session. Running the first three fast and fading through the last four breaks the rule of thumb entirely, because the average that matters is the one you can repeat. This calculator shows the per kilometre and per mile pace behind your target 800 so you can check it against your usual interval pace.
The classic version uses a recovery jog of the same duration as the repeat itself, so a 3 minute 30 second 800 gets a 3 minute 30 second jog. That keeps the session self-scaling: faster runners get shorter recoveries. Shortening the recovery makes the session harder and breaks the comparison the rule of thumb depends on.
Most plans use them as a mid to late block workout, repeated every few weeks so the trend matters more than any single session. A single Yasso workout run on tired legs or in heat says very little. If you want a prediction built on race results rather than one workout, the race time predictor uses a recent race time instead.