Can't recommend anything specifically, but it's probably worth your while to have a look at trailforks and mtbproject for green and green/blue trails.
https://www.trailforks.com/region/austin/trails/
https://www.mtbproject.com/directory/8010446/austin
They may not be all that useful at really deciding which trails you want to ride, but they do get you to the trails and help you navigate them some.
For actually recording rides, and locating yourself on trails, I have found alltrails to be pretty good. One thing it has that Strava really doesn't is entire loops of trails rather than segments that the speed merchants try to ride at suicidal speed.
The Marlin is fine as a starter bike. If you stay committed to riding and enjoy it and want to up your skills and abilities, you can make a more intelligent choice as to a more capable bike later on.
Also beware when researching trails that GoPro video of rides significantly flatten actual climbs and descents and could deceive you into trying something you're really not ready for.