There are many walks that will take you to and along the Great Ouse varying in length, sights and ease of access.
Perhaps the epic walk from source to sea is the 150 mile Ouse Valley Way. Overview of the route.
The Great Ouse Valley Trust have also produced a set of printable leaflets for walks along the trail, with a focus on the stretch from Eaton Socon to Earith. Great Ouse Valley Trust
If it’s the Fens that fascinate, then why not follow the Fen Rivers Way from Cambridge up to Ely where this walk merges with the Ouse Valley Way and continues to King Lynn. Norfolk Council have provided this information
Following part of the Bernwood Jubilee Way provides an interesting alternative by following the path of the Padbury Brook (Twins) which feeds into the Great Ouse. Buckinghamshire Council have produced a set of downloadable walking routes
Progress through the Slough of Despond in the footsteps of John Bunyan. The Wildlife Trust have produced a donloadable trail booklet
Rothschild Way is a fenland footpath commemorating the conservation work of Charles Rothschild. The route is undertaken by a number of people annually in June through the Rothschild Way Challenge event which raises funds for work on the Great Fen. more infomation
If Kings Lynn is too far from the Wash for you then why not continue to walking up to Hunstanton on the King Charles III England Coastal Path
North Bedfordshire Heritage Trail
Long Distance Walking Association
Each local authority publishes the public rights of way within its boundaries:
Buckinghamshire Public Rights of Way
Milton Keynes Public Rights of Way