Oh yes! Of course. I have been writing regularly for over 8 years. I just started to apply that to novel writing though this year. I had been doing creative free form writing all that time via role-playing with friends. We would world build, create characters, write and play out stories back and forth. It has made my writing style kind of weird in that I write from start to finish, editing as I go to help determine where the story is going. I only jump around to tweak and then later in editing to check for flow and obvious errors.
I signed the contract for my first book a few weeks ago. I am loosely working on a follow up to that one, namely plotting it out. I have a standalone that I am working on more strictly for a different publisher. They are interested in considering it for a late June release.
I also adapted a random short piece I had done last year into a novella and submitted that to a different publisher than the other two. Just want to see how each one works and figure out which are the best markets for my stories.
Sounds like you are on your way then. Since you have been previously doing collaborative projects (and gotten it to work) that may be a way to build a reader base more quickly than standalone if you self-publish.
Many many many great writers have other sources of income too. Writing is their career, the day-jobs "are extra income to just pay the bills". Making a career of writing is easy. Just decide. I am a writer, done. That doesn't mean it will support you though. In time, it might.
Here's what I did.
Write, sell, get the dough coming in, repeat. By-and-by cut day-job quota in relation to author income based on average of the last 6 months. Treat it like a business which means either work (all days, even when sick or tired) or go bankrupt. Compare to opening your own store, let's say sell kid shoes in a mall. It needs to be open regular hours every day. That's all there is to it. (I did most of my bill-paying writing doing articles for affiliate marketers at one point. Still do now and then)
Most important of all in my book - never take a loan or whip out the credit card to fund any part of your writing life (meaning buy food, gas, pay rent, buy keyboard.. nothing).
If you are single, no kids, no pets, no loans etc you can squeeze your cost of existence down further if you want to go full-time on a budget. I would not recommend it though because somewhere along the squeeze, discomfort (like sharing room with others) starts to affect productivity.
What most writers fall on is work ethic. You go full-time when the money from the writing can support it (based on AT LEAST a 6 month average) - or if you have xxxxx amount of savings and are comfortable burning it all with a slim chance of making any of it back)
Except for the very few who manage to release a lottery ticket, writing as a career is tough. It's great since you don't have to go to a office, but in time the lack of socializing during work hours gets to some people. It's also great because we can do it anywhere in the world, but that is almost only possible when you are young and before you buy your first house, or after you start seeing the $$$ come in.
It is also great because it is a creative job which let's you express yourself instead of just soldering in a shipyard all day.. but, creativity is like wind - it rarely blows the way you want it to. Writing is like sailing. You cruise along at an angle to the golden creativity most of the time and accept that your output isn't pure genius... but it gets you to port.
Still, I think it is safe to say that establishing a career as a writer is easier today than ever before.