There's a number in your head. It's the body weight you want to get down to for your next race. You know that you will have a much better chance of achieving your time goal in that race if you attain your body weight goal first.
Or will you? It depends on two things. First, it depends on whether the body weight goal you've set is reasonable. I've written previously about setting appropriate racing weight goals. Second, it depends on the means you use to reach your goal weight. There are right and wrong ways to lose weight as a runner. If you choose the wrong way, attaining your goal weight probably won't help you race better. By the same token, losing weight the right way will improve your race performance even if you don't reach your goal weight by those means.
It's important to keep in mind that each runner's ideal racing weight is functionally defined. Your ideal racing weight is the weight at which you perform best. There's no theoretical method of calculating a runner's ideal racing weight that could possibly substitute for discovering it on the road or track. When you achieve the best race performance you're capable of, then by definition you're at your ideal racing weight, regardless of whether that weight is higher or lower than you might have expected.
Body weight aside, you can only achieve your best possible race performance by training properly and maintaining a very high-quality diet. Of course, training properly and maintaining a very high-quality diet will also tend to make you leaner, but that's secondary. The main objective of your training and diet is to improve your fitness. Whatever happens to your body weight as a result of training properly and maintaining a high-quality diet is what should happen. This is one situation where the means are more important than the end.
Some runners make the mistake of thinking that losing weight is more important than how they do it. This mindset may seduce them into eating less in order to get down to that magic number. While this approach is fine for nonathletes, it's not right for runners. Artificially restricting caloric intake while training intensively for a running race seldom turns out well. The body needs sufficient energy to fuel workouts and to bounce back quickly between workouts. If you deprive your body of some of the energy it needs you are likely to see your training suffer even as you lose weight.
Ironically, more experienced runners who know their ideal racing weight from experience are even more susceptible to making this mistake. These runners typically have a definite, experience-based body weight target in their minds when they start the process of getting back into shape after a break during which they may have gained a few pounds. Knowing that they have to reach that weight in time for their next race, they may try to hurry the process by severely restricting their calories. As a result they may indeed get back down to their ideal racing weight, but they aren't the same runner as before because inadequate energy supply has compromised their training.
The bottom line is this: If you train and eat for maximum performance, your weight will take care of itself. But if you train or eat to lose weight, your performance will probably suffer.