Saw Jonathan Marcus post this marathon workout. 4x5,000m + 1x10,000m.
https://www.highperformancewest.com/workout-of-the-day/2020/9/21/4x5000m-1x10000m
the 5,000m reps are at 15k pace and the 10,000m rep is at half marathon pace, 3 minute jog after each rep. I don't think anybody could do this. It's 18 miles of really hard running.