I second HyveMynd's advice. Go ahead and start pounding it out. You will be second-guessing yourself all the time, rework things, start over, and constantly wonder if it's worth doing. But as long as you find it a fun thing to do, go for it.
My two coppers:
Don't keep it a secret. It's very easy to get the notion that you need to safeguard your ideas from people who might steal them, while in reality, stealing RPG ideas isn't much of a venture. It's very unlikely that you will make much money off your game, even if you get it published, so it's also unlikely that someone else would steal your ideas during development stage. Instead, I think getting your ideas out there is a way of "staking your claims". Find other people who also write games. Let them read what you have and request feedback on your work. You will probably find that your ideas aren't all that original after all, and that people have all sorts of opinions about them. You will get criticism. Some warranted and some not. You'll get a bit thicker skin after a while, but you will get much more back from weathering your ideas with like-minded people. You might even get some fans of your game-to-be, or meet people willing to give you a hand.