If you walk to work regularly and go cycle touring you are probably quite fit and are doing all the right things. The only thing I would suggest is a few longer walks at weekends with a pack just to make sure you iron out any feet niggles, and get used to carrying stuff.
However, I speak as one who never does much preparation, just hope that the fact that I do a fair bit of walking will see me through. I usually find the 3rd day a bit challenging, if you are planning an itinerary, I would make that day a bit shorter.
Enjoy it, it sounds a nice walk.