1. Lose Weight
2. Run More
3. Run Faster
Lose Weight: First, we have to assume you have some weight to lose. Obviously if you're already very fit and slim, this doesn't apply to you. But if you can afford to lose a few pounds, you almost certainly will see an increase in speed and endurance. How much? Well, that too depends on how fit and slim you are, or not. Just for reference, if you are a woman of average fitness, age, ability, etc., then losing 10 pounds could net you as much as 10% improvement..
Run More: Second, we have to assume you not running mega-miles already. Most recreational runners are happy to log 10-30 miles a week, while your typical "sportsman" runners are logging 30-50 miles a week. And yes, some fanatics and most elites often log well over 100 miles a week. Fact: Higher mileage has been proven to correlate to faster finish times. How much? Again, that too depends on how fit you are, or not. But for reference, if you are a typical recreational or sportsman runner, adding an extra 10 miles a week could net you as much as a 10% improvement.
Run Faster: Third, again we assume you're not at your peak ability. An unwritten rule says that runners reach their peak potential in about 4-7 years of uninterrupted training. Most runners understand that in order to become faster they need to integrate "speed work" into their weekly schedule. How much? If you have a good fitness base of more than 6 months of running 3+ times a week, you can safely add one day of speed work to your schedule. The most common of these are "repeats" of 400 meters (one lap at the track). Running 6-8 of these at the same speedy pace, with equal time rest between, for 3-5 weeks can net you as much as, you guessed it, another 10% improvement.
The best part of all this is that each of these work in stride with the other. When you lose weight, you can run longer and faster. When you run longer, you will lose weight and run faster. When you run faster, you will lose weight and can run longer.
Try it for a few weeks, see what happens.