Think about it. Terrible weather, but Rupp doesn't want to DNS and get a bunch of crap for bailing before the race even starts, and he certainly doesn't want to forfeit the appearance fee. So Al Sal finds an approximately Rupp-sized runner and throws a mask on him to evade recognition, and plans to have the decoy make a showing and then DNF. The decoy doesn't have to be as good as Rupp. He just needs to be good enough to run Rupp's marathon pace for ~18 miles.
Decoy gets a cut of the appearance fee to 18 or 20 miles all-out while Rupp sits in the hotel room and rests up for London.