"Look at something you wrote at least six months ago (i.e., something you haven't looked at in at least six months), and see whether you can understand it."

I agree with you that legibility is part of maintanability. Just to add a little bit more: legibility does not directly gurantee maintanability. For example, we have this one application that is over-engineered, and the architecture determines that it is very difficult to maintain, even if all of the code are very easy to read and understand, the maintanability is still very poor.

Maintanability at least comes at two levels: coding level and application level (design and architecture).