Only thing I can think of is to get one of those running map apps where you can trace the trails/roads you run to figure out exactly how far you went, & just spend a day checking your location periodically & tracing the trail system as you go. I don't think there's a way to add a trail lines to the pre-existing iPhone map.
EDIT: Scratch that, there's an app that does it for you while you ride:
http://www.trailsoffreedom.com/top-4...p-apps-of-2011
But even so, it will take a day of riding all the trails in the park area to record it onto your iPhone.