Were you cardio or muscle endurance limited?
Yes to both!
I ran out of muscle endurance. The first few hundred feet were easy. Then my muscles got tired, and it felt like I was working them too hard. Then I started to sweat a lot, and I'm sure my heart went up a lot. My breathing definitely went way up.
The DC area, where I live, used to all be more or less swamp - i.e., almost completely level. You have to drive for 1-5 hours to get decent vertical trails - or whitewater rivers, or ski trails. So I've mostly switched to skating, and walking (on flat terrain), for convenience.
The hike we took, the "Gorge trail in "Lower Buttermilk State Park" (upstate NY), is only .65 miles long (each way). It is along a gorge cut by glaciers, and is not flat.
Trail map: http://nysparks.com/parks/attachments/ButtermilkFallsTrailMap.pdf
It only goes up 500' vertically, starting alongside the falls in the picture. Only a 14.5% average grade, though some parts are steeper than others.
It should have been "easy", by the standards of what I used to do, 20 or so years ago.
But a little vertical, and apparently everything changes for me.
(Actually, the group took a longer route down. I think they decided that it was too steep for some people to come down - going down is harder, in some ways. But I didn't make it that far. I went back to the start myself, slowly.)
The trail starts alongside the waterfall in the picture. It's really quite beautiful. But my failure was really quite embarassing.
I guess skaters who jump a lot, including practice on trampolines, jump rope practice, serious strength training, etc., would have the strength and endurance to do this more easily.
For that matter, there is a university stadium within a few miles of where I live, where I could
practice running up and down the steps alongside the bleachers, when there is no game there.