Every writer gets their start somewhere, and, as I’m sure you’re tired of hearing from me, I got mine in forum roleplaying, entirely by accident. I was just browsing topics about the games I had been playing last when I stumbled upon a topic full of people creating their own characters and fitting them into the already established world. I spent an afternoon creating my own addition and found I liked it. I joined in other character creation topics, creating more elaborate and connected backstories each time. It wasn't much longer when I found my first RP that writing became as much a hobby as the games that led me to it.
A friend, made during those times, once called roleplaying, “multiplayer Notepad,” and I'll be damned if that's not how I’ve thought of it since. You still deal with all the work and wonder and stress of building characters, stories, and worlds, but you share the process with others. The friends that you may use as sounding boards for your private works become intimately involved not only in the ideas you bring them, they take on a responsibility to help you execute them within the work. When you're stuck and flailing for any ideas to write yourself, you can always wait and see if anyone else will add to the story before you, to change the situation that has your creativity stymied. More writers bring different styles, points of view, and, importantly, diversity.
But for those positives, there are negatives as well. There will inevitably be drama between writers, but in an RP, that can cause anything from a temporary obstacle to a withdrawal of individual writers to the sudden death of the entire collaborative work. People will vanish, with or without (but mostly without) warning. The odds of actually finishing a story in a group without at least one long hiatus, missing writers, or story implosion are ridiculously small. In almost ten years experience with a fairly dedicated group, I've never made it to the end myself. Not to mention that switching from roleplaying to private writing has been difficult; now if I get stuck in a story, it's up to me to write myself out! (Results on that are… well, nobody's perfect.)
There's a multitude of ways to recapture that feeling of collaborative storytelling (or even experience it for the first time). Not every fandom actively roleplays, but fanfiction is a fantastic way to begin writing in established settings. Gaming helps me find inspiration at times; I still enjoy creating characters in video games and figuring out how they fit into the setting as I play, and tabletop games scratch that collaborative itch. If you're feeling particularly ambitious, you could even find a like-minded friend you could co-author a story with!
Ultimately, how and where you write are up to you. I've been told roleplaying and fanfiction are amateurish and derivative, but sometimes that's the vehicle that brings you to love writing. And in the end, that should be all that matters.