I think this can be measured by the proportion of people who collapse in a heap at the finish line.
In XC skiing, everyone does. Gasping bodies create a significant trip hazard in fact.
So if you finish the marathon less tired than a triathelte or rower finished their event then you didn’t go fast enough
I’d say the number of muscle groups you are using is a factor. So cyclists who don’t use many aren’t under the same kind of load as rowers or skiiers, say. When I cycle (as a non-olympian of course!) the thing that stops me is the pain in my legs, when I run it’s the overall feeling of imminent death. So I can finish a long MTB race with aching legs but feeling generally ok if I’ve nourished myself properly.
That’s why there are lots of week long or more bike races, but there aren’t many week long running races, and those that do exist are for nutters rather than normal top athletes.