I went from running about 50 miles a week to running 90 by the end of the summer. It is definitely a whole new level of challenge on your body. No matter what, you are going to feel fatigue at certain points, nagging pains, and become mentally tired. But nothing is going to replace the feeling you will have at the beginning of the x-country season if you have worked harder than everyone else out there and you show up in prime shape. Try not to focus so much on the number 100. The thing you want to worry about is getting in a consistent amount of doubles and a longer run once a week (maybe on a sunday). The duration of your runs is almost more important than the mileage. If you are consistent with the amount of minutes you run a week your mileage will BE THERE. Try starting off with a week or two of solid singles (maybe 40-50 miles a week). Add a double or two in as the weeks progress. Also, on your long run day, add 5-10 minutes each week. 10 if you can. I would not exceed 120 minutes. Start off around 70 and then build your way up as you feel comfortable.
There is definitely nothing wrong with pushing your mileage over the summer farther than you have ever done before. It's a key time to do so because you are given time to experiment with what your body can/cannot handle. Also, you won't be hammering hard workouts or racing so you should be able to add some decent mileage to your schedule without hurting yourself. Here are some key factors that are going to determine how your body responds to this:
1. BUILD UP. Do not start off running full mileage. Take your week/2week break and then slowly build up your mileage. Once you start to really climb it's ok to go back down a mile or two and then add on again. ( 40-45, 45-50, 50-55, 55-60, 60-65, 55-60, 65-70, 70, 75...) If you need to stay at a certain mileage for another week DO IT.
2. LISTEN TO YOUR BODY. If you need to take the day off, TAKE IT OFF. This does not mean take a day or two off a week, but if it's been 2-3 weeks of increasing mileage and you feel you are fatigued, REST. It's simple...your legs will let you know when they need it.
3. GET YOUR SLEEP. Eight hours of sleep is imperative for training at this level...
4. PROTEIN. I am not a nutritionist but I do know this is key to recovery. After your longer runs you should make sure to get the proper protein and carb intake afterwards (within about half an hour of when you finish). To be safe, I have read that if you train at an intense level it is smart to eat a gram of protein per lb. you weigh.
5. HYDRATE all the time. Because your body will not be used to this mileage, you are bound to get cramping/tight muslces/etc. Water is important for recovery and reducing cramping/knots.
6. STRETCH EVERY DAY. You don't necessary need to stretch before you run; however, never just end a run and then go collapse on the sofa. Take at least 5-10 minutes after each run to gently stretch your muscles. It is KEY to preventing injuries. I am not saying you will get injured, but you are definitely going to experience nagging pains at some point because you body will be adjusting to this new effort.
7. Do not obsess over mileage/pace. Run at a comfortable pace for the most part; on days you feel good feel free to push the pace. Do not start off runs so fast that you are slowing down every mile till you finish only to feel fatiged. Keep your LONGTERM goal in mind- YOU WANT TO SHOW UP TO X-COUNTRY PRESEASON IN OPTIMUM SHAPE, WHICH INCLUDES NOT BEING INJURED/TIRED/ETC.
If you follow these steps you will definitely be way ahead of the game and most likely surprise yourself later in the season with your hard work and determination...
best of luck to you!