I'm not a lawyer, so I can't speak on the legal issues, but at the very least, I would expect some sort of waiver for the participants saying that they understand they are riding on public roads at their own risk.

For distance, depending on your audience and terrain, 30-50 miles is a good starting point. 10 miles or less if you expect to have a lot of new riders and families. Keep the terrain on the mild side so it is fun and not punishment. Century and Metric Century routes only if you expect to have a significant number of experienced riders. When I did an MS150, the century riders were a tiny fraction of the total.

To prove that the money got to where you said, you could have a presentation at the end of ride party to a representative of the charity. Or, if you do it at a later date, issue a press release to local media, including a picture of you presenting the check.