You sound like you've got a good grasp on the situation.
You shouldn't jeopardize your track and 800 goals and potential to help out the cross country team a little more. You're already doing what you can and helping the team, and I'm sure your teammates and coaches realize that XC isn't your forte.
You are a bargain for your team, being that you can help out in XC and track. You're more of an asset than a liability.
As for toughness, breaking down races into sections. Don't think of it as a five mile run, rather, maybe break it down into 4 phases.
My teammate at University was similar to you and his goal in many XC races was to run in a fashion where you only pass people. You run the race so that you're ALWAYS doing the passing and not getting passed. This helped him pace himself and also worked to his advantage at the end of the race when it was time to kick.