Most of the England Coast Path around the East of England is walkable, although several sections are still under development.  There are 15 railway stations along it with some sections short enough to walk in a day.

A nine mile walk, in stages, from Watford Junction station to St Albans Abbey. 

Norfolk County Council's website has downloadable maps of circular walks from the Bittern Line, which runs from Norwich, through the Broads to Cromer and Sheringham.

Walks from and along the lines from Ipswich to Lowestoft and Felixstowe, with downloadable maps. (Linear and circular)

Walks from and along six lines in Essex and South Suffolk with downloadable maps.  (Linear and circular)

Circular and linear walks from three stations in South Cambridgeshire, near the River Cam.  The webpage includes downloadable leaflets with maps and directions.

Walks between the stations along the lines from Ipswich to Cambridge and Ely.

This website has detailed information on hundreds of walks around the country, including many in Hertfordshire, Cambridgeshire and East Anglia.  (Linear and circular)

Walks along the Thameslink lines in Hertfordshire and Cambridgeshire.  You will need to sign in to AllTrails to see the maps and download the gpx files for the walks. (Linear and circular)