Having a properly fitted bike on long rides will pay for itself in comfort and performance. It’s important to first understand the difference between bike sizing and bike fit. Bike sizing determines the correct frame size and a general approximation of the contact points between the rider and bike by lowering or raising the seat and making fore and aft adjustment to the seat rails.
Bike fitting, on the other hand, alters the bike to fit the cyclist’s body so as to maximize efficiency. Bike sizing and bike fitting can – and sometimes should – go hand in hand. Purchasing a bike only to learn the frame size (bike size) is too tall or short limits the range of adjustments that can be done during a fit. A bike fit enables the rider to spend time with a fit expert to find out what contact points can be optimized through the analysis of the relationship between the bike and rider – optimal leg and arm reach and adjusting or replacing parts to make the ride more comfortable.
With that said, fitting a road bicycle works best when you start with the right size bike; or at a minimum a bicycle that is close enough to the size you need.
Sizing a road bicycle is not as complicated as you may have been led to believe, in part due to the reality that a good bike fit actually has little to do with the bicycle per se. Fitting a road bicycle comes down to the contact points (connection points) between the cyclist and the bicycle. The five contact points are: right and left foot, pelvis, right and left hands. So even if your bike is not the correct size frame, as long as you get the connection points in the ideal place you can still achieve a relatively good fit.