Question about Hobbit and Lord of the Rings?
* More than one.
I'm assuming that at the end of the last Hobbit, Bilbo Baggins will still be relatively young, and the war with Sauron and his legions will have started.
* There are like 60 years apart from The Hobbit and the start of LOTR. At the end of The Hobbit, Bilbo Baggins is still in a middle age for a Hobbit. The whole adventure takes less than a year if I'm not mistaken.
* The war against Sauron and his legions (presented at LOTR) have NOT started by the end of The Hobbit.
So what transpired between this time gap?
* 60 years. Bilbo just went back to the Shire and lived peacefully. Hobbits lived just at The Shire and in a few other regions and were not involved in wars or other issues at Middle Earth.
* Frodo Baggins' birth, and the birth of many other characters of LOTR.
* Dwarves established a city under the Lonely Mountains.
* Balin and others established a city under Moria (we saw that city in the first LOTR movie).
* Sauron returned and started a war, Gondor (Boromir was from there) was actively fighting against Sauron and his forces.
Why did it take such a long time for Gandalf and other to formulate a plan to destroy the ring, and find Bilbo again in order to obtain the ring?
* Gandalf did not knew that Bilbo's Ring was the "One Ring to rule the other Rings of Power", the ultimate weapon in Sauron's campaign to conquer and rule all of Middle-Earth. He was not investigating about the ring during these 60 years. It was not until Bilbo made his disappearing act in his 111th birthday that Gandalf realized the ring was something different.
* In the LOTR book, Froddo received the Ring at the Shire, and stayed there for a long time (months?, years?) before start the travel to destroy the Ring, in the mean time Gandalf was investigating the ring's origin and true power. In the movies this was changed showing Froddo to start the journey much sooner.