I took this tour in early November, 2007 and had an incredible experience. I imagine it would be beautiful no matter when you go, but November was a great time to be there. Since it was the off season, there were fewer crowds, but restaurants and shops were still open. The weather ranged from the mid sixties to low seventies, which was a perfect temperature when you're walking a few miles each day!
As long as you're accustomed to walking for a few hours at a time, and prepared for some hilly, uneven terrain, you won't have a problem with the walks. By far the most strenuous walk was the "Path of the Gods", but this is offered as an optional tour, so you can decide whether or not you're up for the challenge.
The Capri optional is not to be missed! The guide was great, and you really get a great overview of the island. Breathtaking scenery as well!