Hey Joe, welcome to the site. Your trainer is right, biking is a great way to get/keep in shape. If you're coming from the addicks dam your first trailhead is going to be at Eldridge.
Click on the Park Map link in the main menu bar (towards the top of this site) and look for the small black and white MTB icon for detail on how to get there. Unlike motocross riding wet/muddy trails is not cool. Enjoy the trails, come back and tell us what you think of them.
