Oh, reading subject again says 20 minute. read it as 20 mile. Sorry
That's a bit more tricky. There isnt a function to create isochrones, assuming that is what you mean, a line/shape showing how far can walk/drive/cycle within 20 minutes?
There are various online tools for createing Travel time Maps,
Not sure if any have a 'KML' export option, which would be the ideal way to import the 'shape' into Google Earth.
Might be able to find something