Hi Mojomcl,
I'm sure you'd manage this walk in a day, even though its over 20 miles it's really not strenuous. We did it in about 6 hours and we certainly weren't rushing, we also has several extended tea breaks and took full advantage of the Dava Way Society's fine hospitality at the half way point. Nothing like a sausage butty and a cup of coffee to revive the spirits!
If they are organising the walk again this year I'd definitely recommend it, not only do you get transport organised and a BBQ halfway, there is also a coffee and scone stop few miles in, and they give you a certificate of completion and medal too! It wasn't over-run with walkers either (sadly, as they obviously put so much effort into organising it and I would have liked to see them raise more money); everyone walks at a different pace so we hardly saw any of the others until the refreshment stops. And of course all the money raised goes to a good cause, ie maintaining the route.
Unfortunately I left my camera at home so I don't have any pictorial evidence of the beautiful scenery you will encounter but it's a lovely walk and we saw lots of wildlife along the way.
Hope you enjoy it as much as we did
Cheers
Jo