I mostly plan my routes in advance, or have a rough idea where I will be going. On some of my longer marathon runs the route was flexible as I needed to be able to add a bit if I hadn't quite got up to distance.
Last year there was a 10 mile event on the same day as the Yorkshire marathon. Does anyone know if this is happening again this year? I can find a link on RunBritain to a race licence, but there seems to be nothing on the Yorkshire Marathon website about it.