and have grand plans for cycling length of the Thames or some such silliness..
Good luck with that – cycling is prohibited for key chunks of the Thames path.
I’ve ridden from Walton-on-Thames to Richmond with my wife (who has a spirit level on her handlebars) and thoroughly enjoyed it. Started to plan a longer epic ride and gave up when I realized just how variable the access is.
I can, however recommend hopping the train to Luton, then riding back via Welwyn Garden City, Hertford & the Lea Navigation – almost all national Cycle Routes, largely along towpaths and plenty of pub stop opportunities. Did this for the first time, recently, and plan for it to be a regular jaunt.
Strava trace