https://www.alltrails.com/explore/trail/us/kansas/elk-river-hiking-trail
The western half of the trail is fairly tame and flat (from what I've read, I skipped that end) but the eastern half is pretty technical in spots, there are a couple of places where you have to scramble a bit over rocks and one place where you have to take off your pack to climb up crack. If you start at the eastern trailhead there is a big, steep hill right off the bat that kind of sucks. I think it's about a hundred foot rise in about a tenth of a mile. But just past it is a pretty neat canyon you kind of maze your way through. I think the roughest part of the trail is the first two miles or so from the eastern end.
I've been on it three times, but I never did the full end to end. The first two times, I took my daughter with me (she was 9 the first time then we went back when she was 10). The first time we started from the eastern trailhead and we only went in about 3 miles then camped and came back the next morning.
If you go down to County Road 5000 there is a place to park and a trailhead sign there where the trail crosses the road. That's where I started both other times and went east from there. I would recommend doing the same, just out and back from that intersection. Going in a couple miles from there to me is the best moderate part of the trail and is a pretty interesting hike. And you'll want to remember your trekking poles.