But they won't have to stop running to use their phones because their shoes will text three friends for them.
Sensors on the road will tell devices in the runners' shoes how far they have got, and will then send the texts.
But so many people go to London to see the race that there are worries the network might get overloaded.
Orange, who are running the free service, reckon that won't be a problem even though 4,000 runners have registered already.
They told Newsround's website that they expected most messages to be sent within seconds or minutes of the runners crossing the sensors on the roads.
England's rugby World Cup winning captain Martin Johnson will start the Mini Marathon at 9.05am on 18 April.
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