Few months ago, a track and field runner Carles Castillejo was involved in something quite interesting: he wanted to qualify to the IAAF World Half Marathon Championship 2018 in Valencia, Spain, to be held in March 24.
He tried to get the minim time in Barcelona on February 11 but he failed due to a flu: he ran in 1h 04:57, while he needed 1h 04:30.
Since he felt he was capable to get the time, he managed to do so in a particular way: he ran the marathon of Seville on February 25, only that running the first half marathon in a fast pace, which would count as official time. As indicated in Carles Castillejo, a por todas en el Zurich Maratón Sevilla Spanish, translation mine:
He will be the pacemaker up to the km 15 of the race. Then, he will try to make the half marathon in 1h 04 to get the classification [to the World Half Marathon Championship]. To make a valid time he must finish the race.
As explained by the IAAF on Seville's marathon report, this is what happened:
The men’s race opened at a frantic pace as the two kilometre point was reached in 5:55, ten seconds ahead of schedule. Once the organisers managed to slow the pacesetters, it was the first pacer, Spain’s Carlos Castillejo, who led the main group through 10 kilometres in 29:58. All the main favourites were tucked in behind him except for Spain’s Jesús España who opted to travel in a second group, well behind.
Castillejo, 39, led the lead group through the halfway point in 1:03:30
After that, he just managed to arrive to the end with an approximate time of 2h 23, which just did not matter.
So my question is: under which circumstances is this possible? What if the marathon was just going down for 21 km and then going back up?
Foot note: he finally could not run with his national team, due to some law interpretation problems. However, he ran the mass race just behind the professional one and managed to overtake most of the runners, as described by the IAAF in Castillejo comes out from the crowd.