One suggestion: Have changes to the route push state to the history. That way you can use the back button on the browser if you make a mistake. Right now it'll totally take you off of the page.
Thanks very much! Personally I find it a little annoying if a site pushes state to the history very frequently. If I genuinely want to get off the site I end up clicking back loads, and then go too far!
I tried to solve this in Trail Router by putting an undo button on the screen (bottom right).
I would just follow how Google Maps works. I think that's how my brain was expecting Trail Router to work, since it visually and functionally is very similar to using the "Directions" feature of GMaps.
One suggestion: Have changes to the route push state to the history. That way you can use the back button on the browser if you make a mistake. Right now it'll totally take you off of the page.