In VIM, how do I break one really long line into multiple lines?

Say I have a super long line in the VIM editor (say around 300+ characters). How would I break that up into multiple lines so that the word boundaries roughly break at 80 characters?

Example:

This is a really long line This is a really long line This is a really long line This is a really long line This is a really long line This is a really long line This is a really long line This is a really long line This is a really long line This is a really long line This is a really long line

to

This is a really long line 
This is a really long line
This is a really long line
This is a really long line
This is a really long line
This is a ...