I'm no expert but if you're a runner already I would have thought that was pretty good by way of fitness, training and preparation. I'm a cyclist too and people do say that cyclists develop musculature that doesn't help in walking, or running for that matter but I have never found this. I assume it only really comes into play if you have the background of a competitive, or professional cyclist. I suppose that I would have to suggest that you try some walks of the distance and type of terrain you intend to cover each day and see how you get on. You should be fine. Walking is easy, it's just putting one foot in front of another! I had worries in the summer that I wouldn't be able to cope with a long distance path after a long, enforced lay off. I wasn't quite up to my previous best but it went OK.