You should not be training for a half on so low an intake.
Training for endurance running, and dieting for weight loss, are not compatible goals.
What is your set goal rate of loss per week? While training you should not be aiming for more than half a pound a week loss. Any loss is going to be marginal at best.
Keep your loss rate low, and keep your fitness 'goal' updated regularly with your additional distance, and you will know that you're eating adequately for your goals.