You are insane and yes it is a stupid plan. The marathon and 8/10K are very different events requiring different muscle compositions and training stimulus. Biggest differences which will hinder your XC season:
1) More mileage but at slower pace - studies show you need to do a substantial amount of running at race pace (or even faster) to be competitive in the 10K, EVEN DURING BASE
2) You can't both increase mileage AND maintain the speed of your runs without risking burning out or injury
3) For an XC runner you should be doing short tempos and light speed workouts each week during base - marathon is about long runs and long tempos. The two CANNOT mutually coexist
The only way you should even attempt this, is if you do your standard 10K base training without bumping your mileage at all and NOT doing any form of long run over 90 minutes, and just running the marathon cold. Anything you do in addition will by definition take away from your XC season.