Do ultrarunners have a huge advantage in the road marathon?

No. Same sport but different distance.

I don't think ultra running really puts anyone at much of an advantage for road racing other than relatively higher mileage training loads, but even then it seems to me that many (but not all, obviously) "ultramarathoners" train basically at the sort of mileage you'd expect for a well-trained marathoner

Walmsley ran well at the OT, but for sure he didn’t have a huge advantage.

Really depends on what kind of ultras they're primarily training for and the type of marathon. But for two people for relatively similar fitness, I'd say mentally they do have a advantage as far as pain tolerance and going deeper into the pain cave. Also in ultras it's rehearsed to hydrate and get calories much more often than roadies do, so that also could have something to do with it as far as training the GI system to take on calories and fluids why running.

If ultra runners had a huge advantage in road marathons all of the pros would be supplementing their training with ultras.

If you think about how Nick Bare has reimagined marathon training, a lot of it just comes down to grit.

So why don't ultrarunners dominate the World Marathon Majors?

Pretty sure the op is implying hobby jogger level of runners and not professionals. As far as professionals, this is due to the different demands of the events/specialization. Look at cycling for example, the best climbers and the best time trials are built different.

Do you really believe they are any tougher than Kipchoge or any other elite runner when it comes to racing marathons? I don’t believe fatigue and pain are the same thing.

For someone like kipchoge that's only out there for a little 2 hours, the demands of a marathon isn't the same as a hobby jogger that's out there for over 2 and a half hours. His body is so efficient at road marathons it would be very interesting to do a study comparing the pain felt by kipchoge and other elite marathoners running under 2:03 for the marathon vs let's say 3 hour marathon runners.

Kipchoge did manage to finish 8th in a marathon in 2020 so I don’t believe he’s immune to a lack of fitness.

Again,I don’t think pain is the correct word unless it’s referring to a sore knee or a blister. When someone’s body reaches a point where it refuses to maintain a given pace, no elite runner is tougher than others.

Your post makes sense. I was in the marathon and the two teammates were ultra peeps--they just took over at 23 when I was dying. I'm new to the marathon distance and they have probably run 4-5 or even more. I know one of them recently did a 40-mile run for fun (8 5-mile loops around his house with friends) this past spring. Another one is doing a 100-miler soon.

So in hindsight, being beat by them should have been expected. The pain cave tolerance you mentioned sounds right. For them, the roads are easier...no elevation changes. They are probably used to running at night, on roots/rocks/trails, with a headlamp, and climbing. But a road 'thon was probably a training run for them.

They are also 9-12 years younger than me, so that may be a factor as well.

I think some ultrarunners probably have a better understanding of fueling compared to an average mid-pack marathoner. Your fueling has to be on point for longer races.

But I wouldn't make generalizations about who has an advantage. Maybe in your example, an ultrarunner breaks 3 hours and beats a non-ultra runner who has faster PBs half marathon & under. There are still tons of non-ultra runners running faster marathon times. The fastest elite runners are still on the track or are topping out at the marathon distance.

Would be fun to see an elite runner take a crack at a real ultra in their prime.

do marathon runners have an advantage in the 1500?

Your post, to me, implies that you're not running at the elite or super high level, when you refer to "part of a running group." Given that, it wouldn't surprise me if the marathoners/ultrarunners are generally stronger than the pure marathoners. In my experience the "normal" people who are also crossing over into ultrarunning are spending a lot more time on their feet than are the pure marathoners. They're doing either longer long runs, or similarly long long runs but doing them back-to-back on the weekends. So, outside of the elite realm, the people you saw might be more ready for whatever pace/mileage you were at when getting passed.

This goes out the window, I think, at the higher levels, because the disparity in training either doesn't exist as much (everyone's doing bunches of miles) and/or the focus is different (broadly speaking, ultrarunners doing more of their mileage with significant vert and at slower pace).

Ultramarathoners are at a disadvantage. They lack the speed of marathoners. Race-specific training is always optimal.

There was a great podcast I listened to on this topic, I believe Jason Koop or Shawn Bearden’s a while back.

The research they referenced specifically discussed running economy and how it was effected by running hills. Results seemed that running hills did not improve economy, only flat did. I don’t think a true trail/ultra runner would do great transitioning over to a marathon with no specific marathon flat training as their running economy would be suffering from all the hill training

Rather than race-specific training, ultra-marathoners don’t have the natural ability needed to be elite on the track or in the marathon. That’s why LR posters like to speculate on how fast the top East-Africans could run ultras.

And East Africans have no skills to finish a 100 miler.