I also picked up a Japanese novel yesterday and am having quite a bit of difficulty reading a scene in which the protagonist is sparring with someone wielding an axe because my vocabulary with respect to hard bits on swords and soft bits on people is rather more limited than the book expects of me.
(If you think the bit about learning Japanese can be compartmentalized away from the bit about learning to persuade or learning what big megacorps value in new products, I respectfully disagree, but you'd probably find the answer "3 years college, 1 year study abroad, 3 years professional experience in (mostly) J->E IT translation, 2 years in software development/management for aforementioned megacorp" more useful.)
I was exactly wondering whether you just learned that language a few years ago and you'd somehow mastered it to this level in a short time or if there were some circumstances that would explain your proficiency better. It's quite rare to find anybody that spends even a moderate time in Japan to know the language at that level.
A good friend has her degree in Japanese and she would not even begin to claim that she's either fluent or comfortable in the language, and that's after 5 years of studying it intensely. To her it's a 'good basis to start learning Japanese', which she says is almost impossible outside of living in Japan for an extended period. It's against that background that I'm impressed, I know a little bit of how much effort that must have been, even if plenty of it went 'as you go' by immersion.
I think that I was looking for exactly the first two words of your reply.
My own 'second mother tongue' (English) I've been learning since I was 10 years old or so and I'm still learning, there isn't a day that I find some turn of phrase that I hadn't encountered before. I've also noticed that speaking a few languages helps to make you (much) more forgiving towards others in a foreign tongue.
Do you come across many foreigners in Japan that have your level ?