HalfHearted,
I live in the St. Louis area, near the eastern starting point of the trail.
I've ridden on it once, up and down the area around Weldon Springs to Marthasville.
It was a cold, gray day in November, and very windy.
Aside from that, it was OK. Probably better with more green foliage and pleasant fall/spring weather.
Lots of farmland, woods, pleasant little towns. Stores and restaurants along the way. I purchased some snacks at a gas station.
The trail is a decent surface of fine gravel or chipped stone. I would say you would need slightly wide tires, especially if there has been a recent rain. There were also a couple spots with loose gravel.
I think your Trek hybrid is probably a great bike for this trail.
No hills. I think this holds true for the entire trail since it is a converted railroad bed.
The trail does cross roads every few miles (in the area I rode) and even on a Sunday afternoon I had to wait in a couple spots for traffic to clear on a rural highway.
I've thought of doing what you suggested: riding the whole length and back. Of course I'm also thinking of doing a mid-Missouri organized bike tour (360 miles in 6 days) and/or an MS150. Someday.
Here's a link to the trail website:
http://www.mostateparks.com/katytrail/generalinfo.htm