I recently wrote to Richard Dealler who has published a book describing in detail, part of the Mary / Michael Pilgrim Line, earth energy lines first identified by Hamish Miller in his book, The Sun and the Serpent (1989). This was his reply (which I failed to ask his permission to publish, naughty me!):
If you are wanting to stay true to the michael/mary lines I think it is very difficult to make this compatible with riding a horse.
I have had a similar enquiry in the past and there are certainly riding routes across most of England which have been identified by equestrian organisations that would connect with some stretches of our route.
We have focused initially on walking pilgrims and thus on the footpath network. Clearly parts of this are also bridleways, whilst other
sections e.g on Dartmoor and Bodmin Moor are open access land where riding is possible, whilst other sections are on lanes.
Beyond Avebury there would also be a route along most of the Ridgeway which would be accessible on horseback.
Sorry that I can’t be of more help.
Good luck with your vision and do keep me posted of progress.
08/05/2014. (St. Micheal’s Day). P.S. Since writing this post, I have met Richard Dealler on a recent Pilgrimage he had organised (post coming up shortly) and still feel guilty about not asking his permission to publish his reply to my enquiry. Returning home, I was on the verge of dumping this post for fear of being found out but decided it was part of my journey so have left it intact because it reflects Richard’s considered response to my question and is a measure of the man himself. Do check out his website. (Click on his name above or watch Rachel Cornish’s short film of one of his guided pilgrimages across Dartmoor).