"Mileage" is only one part of the equation. You can't simply run lots of miles during distance runs and hope to be fast. Distance runs target slow-twitch muscle fibers--not all your slow-twitch fibers, but about 75-80% of them. They don't do almost anything for intermediate fast-twitch and fast-twitch fibers, both of which are recruited heavily during 3 mile/5K cross country racing. In other words, you can train 40-50 miles per week and still have a huge percentage of important muscle fibers remain untrained. To train those fibers (and the energy systems that supply them), you need VO2 max reps (3K- & 5K-paced reps), tempo, hill reps, and the racing you're currently doing. If you didn't do any of that type of training during the summer, then you entered the season without proper background training. You can expect it to take a little longer to hit fast times. It's not about dropping mileage--since "mileage" isn't your primary concern. Your primary concern is correct training that targets all your muscle fibers, the nervous system pathways that control them, and your energy systems. Do that training, then add up the miles afterward--it'll probably be pretty good "mileage," but it's an afterthought to the process itself.