To destory the Ring, but the whole story of getting there was not prearranged. I really suggest you check out the FOTR EE and some of the LOTR explanation books Christopher Tolkien wrote using his father's manuscripts. Basically Tolkien sits down at a type write and has at it. He revisies all the chapters heavily and also the following.
1. He stopped for at least a year in the Mines of Moria.
2. The story with Aragorn/Legolas/Gimli was finished promarily much sooner then the Frodo/Sam/Gollum journey.
3. It took years for Tolkien to finish LOTR.