The biggest thing separating people from their artistic ambitions is not a lack of talent. It's the lack of a deadline.
- Chris Baty
When I saw this quote, I had to stop and think for a moment. I knew the name Chris Baty was familiar. But why? After a quick Google search, I had my "A-ha" moment, seeing that he was the founder of National Novel Writing Month (NaNoWriMo). And the quote certainly fits in that context.
It's easy to set aside the things you want to do or to work on them at such a slow pace that little progress is possible. I'm not sure I'd be quite so dismissive or talent as Baty seems to be in this quote, but I agree with the general point that many people don't chase those dreams because there's nothing prodding them to do so. It certainly was true for me for the first 20 years of my adult life. I've learned to build my own goals and deadlines and it's allowed me to have a very productive year (plus!) of writing since I got back into it last summer.
If you're saying you want to write "someday" then start today. After all, today is indeed some day.