Appleford Station
You can start this trip from Didcot or Culham stations as well, but the route from Appleford is most straightforward one. In Didcot you can stop and visit the railway centre; going from Culham, you'd pass either through the Culham lock or Clifton bridge, by the Barley Mow pub.
We've chosen Appleford as this enabled us to shortcut the great bend of Thames and save up a few miles of walking. Also, this way we got to walk through both Wittenhams, Long and Little.