The distance is forty-two kilometres and a little more, and it does not change for anybody. What changes is how a runner spends it.
Her plan was four hours, which works out at five minutes and forty-one seconds for every kilometre — a number she had written on the back of her hand. She reached the halfway mark in one hour and fifty minutes. Ten minutes ahead.
Her coach, waiting there, did not look pleased. Time saved early in a race is almost never kept. The body is holding a fixed store of usable fuel; spending it faster at the start does not create more, it only moves the moment it runs out. Ten minutes gained in the first half are typically paid back with interest in the last ten kilometres, where the pace collapses and every endured kilometre costs two minutes more than it should have.
The runners who finish near their plan are usually the ones who ran the second half slightly faster than the first, which sounds impossible and is mostly a matter of discipline in the first hour, when everything feels easy and the temptation to push is strongest.
She finished in four hours and nineteen minutes. The ten minutes she saved cost her twenty-nine.