Well I would have thought a brute force hacking program would take forever to crack a sentence... I will put commas and full stops in next time, I will make a whole paragraph! Nothing will crack that!
Your concept is good since it is long and mixes spaces with words where some are capitalized and others are not. However, if your words are all available in a dictionary, then it is possible to use a dictionary attack.
All you need to do, is add a few numbers and one punctuation mark in there and it will be much harder in orders of magnitude to crack.
On a slightly different topic, one issue today, is that some sites have min-max password length constraints. So lets say you have a favorite password system that you use that generates 20 character (letters, numbers and punctuation) passwords for you. Then you go to a site that only accepts 12 character passwords, or one that can take 20 characters but no punctuation.
So you basically need a couple of password generating systems today...unfortunately.