What if you add your own personal coding to the obvious phrase?
Lets say, replace all the letters A with B.
For example "cbptbining finbncial conservbtism mbyonnbise" instead of "captaining financial conservatism mayonnaise"
Will it more difficult to get the key?
I think the point of this entire thread can be summed up as follows:
Give up and use a secure random number generator based on a qualified true random number source of entropy unless you want to lose your Bitcoins.
Yes. Random key it is the best decision, but problem that it is easy forget this random key. You should keep this random key or feed in other place than your mind, so it is additional risk.