A few different perspectives, but all come down to one core factor - learn from the best decorators you can find.
I would say that in theory, the fastest (in a hare and tortoise type scenario) is to learn as Neil did, working constantly onsite from 2 master decorators with the drive to help you be the best you can. If you have that chance, even if it means living on beans, just do it. But in reality, there are not many of those golden opportunities around.
If you go the normal learning on site route, you will meet 95% of decs with a “That’ll do” attitude, and they don’t have a big picture mindset. You don’t want to be learning from people who put a limit on the possibilities. The depth and breadth of decorating as a trade is huge, from knocking out sites to gilding royal households to painting oil rigs, As a mate of mine told me, How can I soar like an eagle when I mix with turkeys.
College lecturers like Russ and Martin have been round the block on site and have a lot to offer, so if you can find a long term course with their calibre at the helm, stick at it for the duration of the course and pick their brains every day. Teachers actually revel in keen students, because that’s what they were (and still are) themselves.
I took a slightly different route in that after doing my C&G Advanced, I went to Art College and did a Fine Art foundation and Interior Design course. It taught me that didnt want to be an Interior Designer, but it also taught me that there is some fantastic work going on around the world outside of a building site and suburbia, and if I could develop some good decorating skills, I knew there was a market for my services, because Interior Designers with discerning clients need people like us 
Then I went on a long journey, but I could only do that because I had a solid foundation to work from, and it didnt hurt to say that I had Advanced City and Guilds of London. To be honest, only ever been asked twice in UK about qualifications, once on a job, once when teaching.
I never went to college to learn about painting boats, but I can assure you I did as much studying and asking and pestering the pros as I could get away with. Passion will get you a long way, because as per the above, there is nothing more exciting for a pro than working with equally keen bods, and it won’t take much to shine. All these TP guys have been doing the same job for 30 years and still love it, as Jase says being in a creative job too, it really is one of the “happiest” professions, if you get the bug - but only you can get your own bug.