Goto maps.google.com and use the Get Directions tab to plan your course.
Click the Link to this page link.
In the Paste link in email or IM edit field, append &output=kml to the end of the url (with no blank inbetween)
Copy the whole URL and paste it into the address bar. Press the return key.
A kml file will now download. Drag the maps.kml file onto TrailRunner or the main window. The course importer will open.
More convenient alternatives:
Convert Google Maps Data to GPX
> GMapToGPX (German)