I'd read Jack Daniels' book. Check out the 5K / 10K training plan.
What you'll see is that it's not just about the mileage. You should build a base of mileage at an Easy pace, then transition into some Repeats, Intervals and Threshold training.
I think the Threshold stuff is most important for XC. So, you should be doing some tempo runs (about 20 mins of running at about mile pace + 60 secs), fartleks, cruise intervals, etc.
Stay off the track--run on trails and soft surfaces. Run more hills than you want to. Do some supplemental training (I like Jay Johnson's GSM stuff).
So, you should by all means slowly increase your mileage, but don't just run a bunch of miles at Easy pace.