Considered one of the top sightseeing routes in the UK, the “Cotswold Way” is a 100 mile National Trail through the countryside from Bath northwest to Chipping Camden. Following a scenic ridgeline, there are some steep climbs, but most of the route is gently rolling on a variety of surfaces. The track is mostly dirt, but some sections through villages follow paved and stone streets. A good running option from Bath is to start at Bath Abbey and can follow the trail north to Kelston Round Hill, for a round trip of about 12 km.
This article is a good overview of the Cotswold Way. Or consult this official Cotswold site for more information about the area. Fun fact: the record for running the Cotswold Way is 19 hours, 3 minutes.