Hi Clair,
   Probable route is thus:- http://www.bikely.com/maps/bike-path/Stretton-under-Fosse-Priors-Marston

At 20 miles It's a bit longer than we would normally do after 11's, so we'll be going directly to the cafe in Stretton from Broughton Astley.  We should be there at 10:15.   The route is generally not too hilly, except at the end, where there's a good climb through Napton to Priors Marston.   We'll decide at Lunch which is the return route; either back more-or-less the same way, or swinging around the other side of Rugby via Willoughby, Kisby and Crick.

-Neil