If players in the NFL that I've never heard of can get $100 million contracts, these big marathons can put out large and deep prize purses.
Make the prize juicy enough and even Kipchoge would find a way to get on the starting line.
Give the winner $500,000 and go all the way down to $1000 for 50th place.
Throw in a few bonuses in for leading at 10k, half, and 30k.
And don't separate the elites from the masses.