So I'm still fairly new on this thing (little over a month at this point), so I'm by no means an expert.
Right now I'm comfortable at the 15-25mph level.
I prefer riding in streets, but tend to stay on back roads. If I have to be on a busy road, then I stay on the sidewalk. As sidewalks in my town can tend to be very bumpy and doesn't make a comfortable ride!
I've seen some modification where people will put a seat on their wheels. This looks way more dangerous than standing. I can at least jump off if things get dicey! My feet have built up a tolerance where it doesn't really affect me to much to ride for say 10 miles without having to step off and rest the muscles in the feet.