Hmm, never heard of "Walking Festivals"
From the Guardian travel feature on the "Heart of Wales" train route and newly inaugurated parallel walking trail ("All aboard a new walking trail across Wales"):
Kendal Walking Festival
The beautiful Lake District town is the base for exploring the area with a programme of guided walks for all levels, 10-12 May
Suffolk Walking Festival
From a dawn walk to a twilight hike, the Suffolk Walking Festival has a huge choice of short strolls and long yomps, 11 May-2 June
Isle of Wight Walking Festival
Enjoy spring on the Isle of Wight with over 100 walks over two weeks, taking in scenery from the chalky east cliffs to the rolling hills, 4-19 May
-- Heart of Wales Line Trail
-- Heart of Wales Line Trail friends organization
Interestingly, the trail existed for the most part before the formal creation, which was pushed by locals and the development was funded by Arriva Trains, the operator of the train route.
Separately, the organization planning the HS2 high speed rail line in England has scuttled the previously planned parallel bike and walking trail ("Scrapped HS2 bike path 'five times better value than HS2 itself'," Guardian).
Apparently, walking festivals are more about hiking:
-- "The joys of the UK's best walking festivals," Guardian
-- Guide to Walking Festivals in Britain, Walks Around Britain
-- 2019 Map of British Walking Festivals, Ordnance Survey
but in the vein of the Jane Jacob's Walk events the first weekend of May, there's no reason to not repositioning city walking tours as part of an ongoing festival.Arlington County Virginia's WalkArlington program is another example.
-- Walkabouts publication and maps