A method that contains a high number of lines of code has a number of problems:

It can be difficult to understand, difficult to check, and a common source of defects (particularly towards the end of the method, because few people read that far).
It is likely to lack cohesion because it has too many responsibilities.
It increases the risk of introducing new defects during routine code changes.

Break up long methods into smaller methods by extracting parts of their functionality into simpler methods, for example by using the 'Extract Method' refactoring from [Fowler]. As an approximate guide, a method should fit on one screen or side of Letter/A4 paper.

M. Fowler, Refactoring, pp. 89-95. Addison-Wesley, 1999.