How far should I run before I sign up for my first race?
This depends on the distance of your first race. If you can comfortably complete the total mileage during training, then you're ready to run that distance as a race. So if 6 miles is relatively easy for you, then you're cleared to race a 10K.
The only exception to this rule is the marathon. Since this race is so long, you'll probably never run as long during training. But beginning runners should complete at least a full year of consistent running before you attempt a marathon. It's a tough race.
How long should my first race be?
Like the last question, this depends on your current fitness level and how comfortable you are running longer distances. But a good general rule to follow is to start with short races and gradually introduce longer races as you get in better shape.
Here's an example: After two to three months of consistent running, sign up for your first 5K. If you like it and are ready for a bigger challenge, sign up for a 10K after another month or two. After about six months of consistent running, you might be ready to tackle a half marathon if you haven't had any injury problems.
Keep in mind that every runner is an individual and these principles can change depending on your background. Nevertheless, this advice can help get you started on the right foot.race.