Major in something you'd like to teach - don't limit yourself to Phys Ed, by any means. All of my XC coaches were math teachers, and most of the coaches in our league now are math teachers (go figure). As a walk-on, I'm the odd man out.
It would probably help to start at a school that doesn't value XC much - maybe they have a non-runner coaching out of desperation, and would be interested in a real runner coaching their kids right away, instead of you waiting for a good coach to retire.
If you're in Cali, check espn/dyestatcal for info about open coaching positions - it's even linked on their front page.
You may not get lucky and land a coaching job with your first teaching job - my favorite coach waited years for his 'dream job' to open up at a high school on the Central Coast of California.
PS: You can start padding your resume by looking for an assistant coach / volunteer position at a high school close to your college. Getting your foot in the door can help - maybe a rival school sees you coach, and you let your intentions be known, and when they have an opening of their own, you're already known by the coaches in the area, which can help you get a job.
And it doesn't hurt to know someone. If you have a 'friend of a friend' who knows about an opening, chase it down.