This is the ability to forget all about the blisters, shin splints, ITB injuries, bleeding nipples and bruised and missing toenails.
It generally affects runners a few minutes after they cross over the finish line when they miraculously forget all about the fact that they woke up in the pitch dark, drove for hours and stood in the icy cold wearing skimpy clothes on the start line, they forget about the aching muscles, the uphills that go on and on and on and the km distance markers that seem to be placed further and further apart the closer you get to the finish.
The runner who swears never to enter another marathon again will happily sign up for the next one minutes after crossing the finish line. The runner who says they will rest the minute they feel an injury coming on will continue running until they cannot run anymore. They will enter the most nasty trail runs with killer climbs and muddy slopes despite the tears from the year before.
Runners amnesia is in fact the only thing that ensures that there are still runners out there and that the event organizers dont all go bankrupt as we continue to enter event after event blissfully unaware of what is in store for us.