Start riding now. Commute to work if possible, and do longer rides on weekends. You can ride a tour without training, but it will be much less enjoyable than if you are in shape. Riding will help you in a number of ways, in addition to getting more fit. It could help you lose weight. It will help you get your bike adjusted and set up properly. It will help get your butt used to sitting in a saddle for extended periods. Longer rides on weekends will probably do the most good, but rides of any distance will help.