You want to: Set a personal record (and smoke the competition)
You should try: The same time, all the time
Ever notice how there are workouts when everything just seems to click and you feel almost bionic? That's how you want to feel during a competition, and you'll have a better chance of that happening if you work out at the same time every day. Whether you choose to train at 6 a.m., noon or 6 p.m., the timing must be consistent so that you can rule it out as something that could affect your performance. Then you can act like your own coach, analyzing your workouts and looking for rogue factors that tend to throw you off (your pre-race snack, your stress level, the amount of sleep you got the night before). You'll have your routine down by race day, and you'll know you did everything you could to put your best self on the starting line.
Next: 8 ways to get a better body in 30 minutes or less