Hi Big_foot, I have suffered with this for 2 1/2 years now. It first came on on an easy flat walk up the Sefton coast path, firstly feeling like a bit of a hotspot on the heel, then becoming quite painful, and eventually leading to me heading inland and getting a train back to my start point. In the ten minutes on the train my foot locked up so that I was hardly able to walk at all. Some physio and exercises helped, and got rid of the worst of the pain , but I still have to manage it. We are all different, and unlike Mel I find that ice works for me - when I get back from a walk I ice it for about 20 minutes, then have a shower and then do another 20 minutes icing. I find that immediately after icing the foot is stiff, but that this wears off quite quickly and then I am pain free. I use ibuprofen gel sparingly at other times and I also ice it occasionally when I've not dome much on it.
With regards to footwear, my physio advised me to wear boots as much as possible, even in every day life, as they support the heel. I did buy some lightweight walking boots but don't wear them so much now as I found they had the same effect as Mel describes below, but I do wear boots in the hills and don't suffer the same issues as Mel describes - mine are Meindl Bhutans and I find them very comfortable, but again, I think it is a matter of finding what works for you. They are certainly not boots I would wear to work/around town!
I have accepted that I will have to put up with this, and it does impact a little on multi day walks, but I still manage to get in the hills virtually every weekend (touch wood). My physio was happy for me to go hill walking, he just gave plenty of advice about management, and I go back if the problem becomes acute rather than chronic. The biggest change I have made is that I now walk a little slower, as this puts less strain on the tendon.