I'm a native French and Spanish speaker. I also understand Italian and Portuguese. I took six years of English in school/high school, and ended with a very average (read: bad) level. After high school (at 17) I couldn't afford traveling or taking courses, so I decided to learn on my own watching TV shows, and I quickly saw pretty good results. In a matter of months watching
Friends, I quickly got much better than what any of my former teachers would have thought possible.
The first time I went to an English speaking country (the US), I was 24, and I was pretty much fluent. After that, I spent a year in Australia, which also helped a lot.
The other day I took this test, and got a 22000 words result:
http://testyourvocab.com/.
Of course, it comes at the price of some serious effort: I had to watch a lot of TV!
Try to get used to different accents. At first, I focused on US English, and for years, the UK accents were mostly unintelligible (and I still have trouble in a few cases)
Compared to German (I took one year of it in HS) I'd say it's very easy (again, in HS, the teacher was very grammar-oriented, and I didn't pay much attention). Considering I can barely figure out simple things like a verb in a sentence, grammar and rules aren't the best way to make me learn.