You don't have to be a skilled cyclist to bike through wine country, and you don't have to be in perfect racing shape. But if you're just a casual biker who wants a pleasant trip that includes visits to wineries, be realistic when you plan your outing.
Whether you're planning your own route or going with an arranged tour, don't take on more than you can handle. Choose terrain that suits your abilities, and don't try to cover more miles in a day than you can comfortably handle.
Maybe you're not sure what sort of climbs and downhill plunges a particular route involves. Maybe you don't know how many miles you can reasonably cover in a day if you take time out to visit some wineries and relax. That's where experienced tour companies can help, whether you choose a self-guided tour or a fully supported one.
Tour companies offer various levels of trips, including those designed for people who haven't done much biking since they were kids. Don't book a tour on the basis of what you read in a brochure or online. Talk to a guide who has biked the route. Be honest about what you want and what condition you're in.