You'll see the track marked A to B on the mapboard at the entrance to the track. Your best best for a fun ride is to go out and back on it. C to D will test you - it's steep and gnarly in places.
You won't get lost on A to B if you understand that there are two more or less parallel tracks that cross here and there. One is a rough horse-vehicle track; the other is a biking-hiking track. You start on the horse-vehicle track and come back to it from time to time. Whenever you're on the horse-vehicle track keep your eyes peeled for where the biking-hiking track branches off because it's much more fun - nicely contoured and benched (most of the time). There are signs but they are easy to miss. The biking-hiking track was built a century ago and then neglected so has slipped away in places, but has little bridges where needed. It's narrow so you have to squeeze between trees sometimes. And there are roots here and there but its suitable for intermediate riders who are not in a hurry.
After about 5km you'll come to Point B. Sharp left is where the long distance walkers on the Te Araroa Track come up to share the biking-hiking track with you. Sharp right swings back the way you came - it's the horse-vehicle track. So this is where you turn around, homeward bound. If you want to ride C to D and back it branches off shortly.
The Waitomo Forest Track is a mini-adventure back country ride on a well contoured hiking track in regenerating native forest. It's quite fun. You're better to stick to the hiking-biking track rather that making a loop of it by riding back on the horse-vehicle track.