I suppose I can use this as a chance to re-introduce myself and comment a bit.
I was introduced to the language through the early 90s when importing games from the back of video game magazines in America. Then I got into the fansubbing scene. Throughout all of that I had zero interaction with native Japanese people except through very random internet experiences. This was early internet days so there weren't any resources back then like there are now. I didn't formally begin to study the language until early 2000, but even then just from all the years of media consumption I was clearly above beginner. Then I did a year abroad at Sophia. Then I came back and realized that I would never become proficient from simply continuing school studies. Then I did my 3 month pass JLPT intensive study (found in the op).
So basically I had like 6 years of informal Japanese study, about 2 years of university and even study abroad experience, and even then I felt I was barely intermediate. I think all of those years gave me a huge advantage in terms of understanding the culture and the 'method' of Japanese, but I didn't grasp the technical aspect.
After those 3 months of study, I was basically never the same again. In those 3 months I made the past 8 years meaningless. There is a very huge difference between passively consuming shit like games or anime for years, and actually treating study as a job and treating it with respect.
I guess my point is, spending ~8 hours a day for 3 months in my room was the hyperbolic time chamber and I came out of there able to communicate in ways I never knew I could. I obviously wasn't super advanced or fluent or whatever metric you want to use, but I pretty much secured my future and career from the abilities I learned in those 3 months. Meaning I could clearly pass a corporate Japanese interview with no prior work/interview experience in Japan just from my self-study.
Short answer to your question: It's very, very doable but you really, really have to work for it.