Oh, I am sure you ARE in good shape. Cycling just uses your muscles differently than you are used to, so you just need to get them up to speed, so to speak. The cardiovascular fitness that you have already built up is going to help you a great deal, and will allow you to train harder to build up miles/speed faster - if that is what you are interested in. Good luck with the training, and more importantly, with your event! Can't wait to hear about it.
