As someone who wasin basically the same boat, allow me to offer some points of advice.
1. You are already doing a good move by making your own setting. Once I got the ground rules down for my setting, and what made it mine, rather than another generic setting, I found it much easier to do the rest through simple Q&A here on the boards. Maybe you don't need it but other people's questions helped a lot in figuring out where I was going with my world.
2. This is not a fragile game. Once you have your own world, and can understand the internal consistency, you can do a lot of stuff on the fly. One of my players wanted a new bard subclass, and I had the basics done in about a minute, because I was immediately able to know where it fit in my world. To sum it up, do not worry too much about if your races get over powered or not, it tends to bend quite well before it will ever break.
3. There is no such thing as too much to do, only too little time. Maybe see which of your races/classes/spells your players are interested in playing for session 1, and make sure those are fleshed out enough, mechanics wise, that you players can play them. You can work on the other things in the background, and in between sessions.
That is all for now, good luck.
EDIT: 4. Also, be prepared to be flexible. I threw 30 wolves at my party, fully prepared for them to either run or rally nearby guards to help in the fight. They instead started burning down trees, while the wizard played fire man to keep it from turning into a full blown forest fire. Players come up with some crazy ideas, and it might help ahead of time to decide what will or will not work to an extent.
For example, with the wolves, they tried scaring the wolves away with fire, which makes complete sense. Then the rogue jumped onto a burning branch to chop it down, so it would fall on the wolves, so getting a bit trickier. Then the Wizard used Jump to try and crush a wolf, by jumping thirty feet and landing on it. Now in the realm of complete improvisation, at least as far as I was concerned. I just wound up ruling they both took the falling damage he would have taken from 15 feet up.