Not all of the sustrans routes are roads, roads are used where cycle paths aren't available.
If you have a rough idea of where you want to go you can string places together with the sustrans routes and check out local council web pages for more detailed info on them. If you're planning to cycle around a particular area Ordinance Survey maps are the way to go, these are maps designed for walkers etc and are very detailed and will show cycle routes, have contours so you can judge hills and even have pubs marked