I am 55 years old, in decent shape, don't walk or bike a lot but this was pretty easy walk. The only thing easier would be flat pavement. Grade is 240' per mile which is not really noticeable. no ups and down, just even grade downhill going downstream, and slight uphill going upstream.
It might have been somewhat exerting if I had to pedal a slight uphill grade on a bike (from New River upstream along Mill Creek to the trailhead). But I only bike on flat pavement! So I walked it. Going down I could have slowly coasted all the way. This would be an easy mountainbike trail for even moderately mountainbikers, but I would not take a road bike with narrow tires and expect a leisurely flat ride in a middle or upper gear. Mostly hardpack gravel and dirt, a couple of damp places but no real muddy puddles.
Some very scenic waterfalls, one big one about 20' high, can wade out in the stream a few places. Some interesting sedimentary geology and one abandoned coal mine entrance. It would be very pretty in the fall with the change in leave color, but some places the leaves would partial block the view of Mill Creek, at least for picture taking.
I went Sunday April 17 2016 and the leaves were just emerging. On that day there was a large tree, about 18" diameter, across the trail about half way (1 mile mark or so). It fell from a cliff above so it was across at an angle, not laying flat, so you had to climb over it, or crawl under it. You could not ride a mountain bike across it but you could lift it over.
I started at the top and walked down it talking my time and taking pictures, took 60 minutes to go the almost 2 miles. Coming back, kept a brisk exercise pace and didn't stop and made the 2 miles back "uphill" in 40 minutes. You can also walk just one way if someone will drop you off and drive to the other end to get you.
Bathrooms and parking at each end (a road follows the other side of Mill Creek), drinking fountain only at bottom. Plus at the bottom where Mill Creek empties into the New River there is a small gift shop, refreshment stand and nature center. All were not open as of April 17, might have been too early in the tourist season.