It looks like there's a walk from Great Chesterford to Linton, partly along the Ickneild Way. Should be about 5 miles, or perhaps a bit less, but with hills (114m!). If we're feeling keen enough there's an almost circular route back, and if not there are buses every half hour back from Linton to Cambridge that stop at Addenbrookes as well as in town (assuming we get the train out).

What do you think? Or do you have any other ideas?