I have no idea why this trail is called "Endurance Loop." The trail is only 1.25 miles long for starters, hardly a test of endurance. If it were all uphill I might buy the name, but it is not. That said, it's a nice little trail, in fact it is one of my favorite sections of the Explore Park
This trail begins at a well-marked fork to the left from the intermediate trail, with some nice little climbs and descents, curves, and the usual roots and small rock gardens to navigate. A little more than halfway through the trail is the most difficult part: a longish, fairly steep climb through 2 switchbacks. I usually have to push my bike partway up this section, but I'm an over the hill former midwestern road biker who has only been riding in these mountains for a year, so don't give my experience much credence! Past this section it is back to fun riding the rest of the way.
Not only is "Endurance Loop" not much of a test of endurance, it is not even a loop! The trail ends at the intersection of an old logging road and the beginning of the inner loop trail. If you believe the name and think you've tested your endurance you can turn right and ride up the logging road to where the inner loop and intermediate trails intersect, or you can continue across the logging road onto the 1/2 inner loop trail which will take you back to the same place. From here you ride the intermediate trail back from whence you came, passing the starting point of Endurance along the way.
I often warm up on the beginner trail, then ride down intermediate to Endurance, take the inner loop trail back to intermediate and ride back to the parking lot in just under an hour.