[brofallon]

Trail runs are different for a few reasons. First, long endurance runs take way more time, maybe 16-18 hours for top-tier athletes to do 100 miles, while a fast marathon isn't much over 2 hours. Also, there are typically lots o really big hills, which require more leg strength to run up and down. There's a lot of additional challenges to running in the mountains (rocky / uneven terrain, snow, river crossings, rain, darkness, etc), the pace is typically much slower. Even pros don't often run much faster than 8 minute / mile pace in a long ultramarathon, while Kipchoge can run most of his miles at 4:30 pace - as long as it's a flat road and it'll all be over in a couple hours