Did this with two friends in early August 2017. Had a fantastic time, although this was our first major backpacking trip so we all carried too much food/gear and our packs were all 50+ lbs.
Compared to local Detroit trails, the Greenstone is physically demanding, but compared to most mountain trails, it really isn't that bad. The terrain is never really flat - it's constantly either up or down slope, but it is rarely steep.

We had a lot of fun stargazing and seeing the Perseids meteor shower. There wasn't much light pollution at all, but there were lots of high altitude airplanes flying by, which kind of ruined the remote feeling while stargazing.

The trail goes through multiple types of forests and it goes from easier to harder as you head from West to East.