I just incorporated the Katy into my last trip, rode it both ways, from Clinton to Machens and then back a few days later.

Used 32mm vittoria rando crosses. There were a few soft spots where I had to put more effort into it, I weigh 220 with a sixty pound total rig. If speed is not your goal then a wider tire would be better. I was looking for a compromise with a couple hundred paved miles or riding on each end, so stayed with 32s.

The biggest thing to look out for is black walnuts on the path, they were just starting to fall when I was coming back on the 12th. They can be more pesky than horse droppings. Should not be a problem in may though.

Random stuff on the trail:

It is a lot easier going west to east.

There is no camping really from Herman to Machens. I pulled that leg all in one day both ways, but that meant that where I stayed in Machens was not really approved. There is nothing there at all though, and I got there at dusk. Be advised that if you do "rest" there for the night there is a train switch right next to it, there will be trains idling and thundering past all night... I rode on to St Louis and there really is nothing at all there, so if you do want to do it end to end probably figure on staying in St Charles. Its a neat town.

Biggest advise is to use a dry lube on your chain. I messed up and ran some wet, took a few years off my drive train with all the dust it picked up. Fenders are best, IMHO. Even with them things can get really dirty.

Coopers landing looks really horrible, but it can be a fun place to stay.

I have run into people doing twenty miles a day to 100. Road bikers in huge groups, cross country riders just using the trail as a route, all sorts. The other riders you meet can be interesting.