Hmmm....we've done self-guided, but on touring bikes and at a sight-seeing pace. It was with Rondonee ( http://www.randonneetours.com ). While they're really good at customizing the itinerary, I'd say it unlikely that they could plot you more challenging routes. They're more of a company that plots vacations that use a bike.

On the challenging cycling route, there is Cycle Italia ( www.cycleitalia.com ). But, they're a full-service group. They're adding a do-it-yourself, but you might not get to see as much of the country that you want. If you want great routes and challenge, Cycle Italia can provide.

Although you probably have already figured it out, what you're looking for is "self-guided" as the keyword. We've looked into some of the companies
( e.g., http://www.pure-adventures.com ), but haven't ridden with any of them. I really enjoyed our self-guided trip through the Dordogne and look forward to doing another, but, like you, I want more mileage and challenge.

Good luck.