Ton – do the podiatrist thing. Painful heels are what got me into cycling – it got too painful to do multi-day walks. I could give you chapter and verse on this 🙂
A podiatrist will be able to analyse your gait and can refer you for scans, blood tests etc. They can also come up with proper, supportive insoles.
Having said that, aim to get the most comfortable, cushioned soled shoes you can buy, and be prepared to change them regularly as the sole unit gets compressed over time.
I’m currently using some Sorbothane insoles in my walking boots and they do make a slight difference.
Oh – and consider those weird MBT shoes. When I was working in the shop – and therefore on my feet all day – they were what made it possible.