Break down the year into five training cycles: Summer, XC, Winter, Pre-Competetition Track, Track.
Figure the number of miles you want to run for the entire training year (let's say 3500, average of 70/week)
Then break down what percent of your annual mileage will be run in each of those cycles.
Summer: 24% of 3500mi = 840 miles
XC: 20% of 3500mi = 700 miles
Winter: 25% of 3500mi = 875 miles
PreComp: 18% of 3500mi = 630 miles
CompTrack: 13% of 3500mi = 455 miles
Each training cycle is obviously ten weeks long, so 1/10th of the volume in each session is your weekly average. But don't just run the same mileage every week. Work it out so that there are two up weeks and a down week. For example if you're in the XC phase and you're AVERAGING 70 mile/ week, set up a 3 week cycle where you go 75/75/60. The third, lower week should have a day off.
Follow the same plan of two weeks up, one down for most of the season. The last few weeks of track should be the only exception.
Hope this helps,
--trackhead