I think Everest is a lot harder, of course you mean everest using Oxigen, cos without Oxigen is not even possible to compare
the two things, is like to go on the moon, or just do a peaceful strolling in a sunny day (marathon).
Manuela di Centa an Italian cross country ski olympic champion from Italy climbed everest with oxigen , and after that
she said that she had never suffered like that in her life. I mean the cold, are enough to make the everest a lot harder.
then if you consider bad weather, wind, the picture can get worse in a moment.
Without Oxigen is another dimension. The Marathon can be hard, but you are at sea level, on a tarmac road, with a confortable temperature. While on the Everest you got to go up where Jumbo fly passing through giant serac, ice fall , snow,
strong wind, and cold.