I just graduated with my bachelors in mathematics last year, so I have little experience in writing huge, very involved proofs. The longest proof I've ever written was about 10 pages, but it wasn't even 10 pages "long" in the way I'm talking about, it just had many cases.
When I have a general idea of a proof, I sometimes prove the theorem in one attempt. Rarely, I completely have a rock-solid proof in my mind before writing anything. Other times, I simply deduce "what I can" and hope that it leads me to my result, or that it at least sheds light on additional facts I need to investigate in order to prove the theorem.
In the last case, sometimes all of my deduction was quite useless in showing the result, and so I start over and take the proof in a different direction, or research theorems that may help me.
Then, I hear about these incredibly long proofs done by world-class mathematicians, and sometimes, I even hear of extremely long proofs (thousands of pages) that were contributed to by many mathematicians working together.
My question is: how are these long proofs "planned?" Is a general outline formulated after much discussion? Surely it is not feasible to get 200 pages in and realize your proof is not leading to the result. How do you embark on a 1000-page journey for proving some theorem? I suspect there are meetings, preliminary work, plans, and things like that.
Is there a general technique for planning these long proofs that I should familiarize myself with as I continue to move forward?