What's the most you ever wrote?

For some strange, unknown reason I've been getting a weird writing itch lately, and I have a structure of a story figured out already and all that. I used to get great grades on creative writing when I was in high school, so I feel like I could do it, but it's really tough knowing where to start. It got me wondering what the longest thing is that I wrote in any vague semblence of coherency. The latter cuts out the notes on the world I've been building for the past 26 years as they're scattered and drafty.

It dawned on me, though, that the most I ever wrote was back when I had penpals in my teenage years, particularly my cousin. I even wrote a 100 page letter to her once! We started off small and eventually, by responding to each other's letters while also adding in stuff of our own, we ended up writing longer and longer letters. I'm pretty sure the last letter I wrote to her was a literal school exercise book instead of basic pages. I still have binders full of feathers from my stepcousin, cousin and my cousin's three friends from back in that era.

But for creative writing, I fear I've never really gone longer than I had to for a high school activity or exam. Fingers crossed this little itch will turn into something complete, but knowing me, it might take some years to figure out how I want it to be.