On a long proof

Not only true, but not unique. The abc conjecture has a recent (2012) proposed proof by Shinichi Mochizuki that spans over 500 pages, over 4 papers. The record is the classification of finite simple groups which consists of tens of thousands of pages, over hundreds of papers. Very few people have read all of them, although the result is important and used frequently.

Math can be very difficult.

There are famous long proofs, like that of the Feit-Thompson theorem, whose initial proof took 255 pages of very intrincate arguments, or the classification of quasi-thin simple groups done by Aschbacher and Smith in 1221 pages, or well, the whole classification of simple finite groups —which is estimated in the tens of thousands of pages— of which the result of Aschbacher and Smith is a very small part.