Although Google Maps isn't great for navigating offroad, it can be handy to see where things are before or during a ride. It's possible to export data from Google Maps, you might also find the data on the Downloads page useful if you're after importing the information into another software package or app.
Cornwall Council Byway data from FOI:
https://www.google.com/maps/d/edit?mid=1o1VRyy4veNtl3bRQWNo-A9Zh9JJMQpdF&usp=sharing