The actual process of coding takes percentually less of a software project's total time as project size grows - regardless of the development methodology involved.
As complexity increases, more time must be spend on design, planing, unit testing, documentation and refractoring than in simply "coding on the fly".
As a project's size grows it is my personal opinion that an interative approach to software development must be taken - it will guarantee a better quality project and ensure a higher success rate.
Unit testing should be an integral part of software development and you should become familiar with one of the popular test methodologies.