Ideally both elite and club runners want to maintain an even pace throughout the whole race. The chart shows average values for male runners but elite runner Butler would aim to run five-and-a-half minute miles.

"Realistically you will slow down in the final third and hopefully pick it back up again as you get closer to the finish," she says.

Club runner Fagan wants to run a 3:15 marathon.

She says: "I ran my first marathon amongst 500-600 runners so the sheer numbers competing in London may make beating my personal best more difficult."