Once you choose your date and location, the very first thing you need to do before you tell anyone about it is start the permit process with the city. Races usually require shutting down streets and detouring traffic, so you must have permission from the powers-that-be to move forward.
You'll also want to create a budget for your event to ensure it's financially viable. There are a lot of people involved in making a race happen, and a lot of them need to be paid, like police, medical staff and your Web designer. Then, figure in costs for things like port-o-potties, water, tents, cones, T-shirts and signage. If your goal is to break even, you need to make sure your entry fees cover your costs. If you're looking to make money on your event, you might want to consider getting some sponsors. You'll also need to come up with a marketing plan to attract runners. This means creating a thorough Web site, as well as printed materials to distribute.
Next, it's time to focus on the logistics of the race. You need to make sure your course has been accurately measured and marked and that you know all of the places where water stops, signage and clocks should go along the course. You'll want to plan for a seamless registration process, so that runners can check in quickly, get their numbers and be on their way. You'll need to make sure you have plenty of volunteers to help with things like manning the water stations, running the registration table and handing out T-shirts at the finish line. It's important to clearly communicate your plan to all of the people involved to ensure things run smoothly. You'll also want to make sure they stick around to help with the breakdown when the race is over. Maybe even consider having a fresh group come in to help with the after race work, since it's likely that everyone who's already there will have already had a long day.