Erwin Park has an official trail head starting point. However, the trails make 2 loops from this point and didn't, in my opinion, come together too well. So, I recommend a different route. You can do the route either way depending on whether you want to do the easier sections first or do the harder sections first. The beginning of the harder section is actually a good warm up so I recommend the harder section first. If you do it this way, you will be riding in a clockwise direction. From the parking lot, ride directly into the trees that are immediately adjacent to the parking lot. It won't take you long to find the trails. Turn right and you're good to go. This early section has the best section of woop-dee-doos as it meanders through dense hardwoods along a creek bottom. Once the trail finishes meandering through the trees, it enters a big field. This provides an opportunity to turn on the after burners and really get the heart pumping. The trail makes its way to the park boundaries and runs along a fence for a short distance before heading back into the trees. Then the tight single track starts up again. There is one more section of field and one more section of riding in the trees before finishing the first loop and reaching the main trail head. This first section has some very tight and fairly technical sections that require careful negotiation or you will end up hitting a tree or having a stump or root send you for a spill.
The main trail head is at one end of the parking lot so you can end the ride here. Or, you can merge left onto the easier loop for a few more miles of fun. This section of the trail is not nearly as technical or tight or twisty as the first section. There is also a bigger percentage of the trail in fields. So, this provides a good opportunity to really get some speed up and catch some air on several small jumps. Toward the end of this section, you enter the overflow parking lot which is just a big, grassy field. Shortly thereafter, you cross the road you drove in on. Follow the road to the parking lot and you have completed one giant loop of around 8 miles.