There is no single entrance, which is the main reason visitors never get on it. Pick an access that matches what you actually want.
From the Nature Center area in Gulf State Park. The best first visit. Parking, staff who will tell you what is currently nesting or basking, and short loops that get you into woodland within minutes. Suits a family testing whether anyone enjoys this.
From the campground. If you are staying in the park you can ride out of your site. This is the single strongest argument for camping here over booking a condominium tower on the beach road.
From the Orange Beach side. Access points off the eastern end put you into the wetter, swampier habitats first, and the Orange Beach segments tend to be quieter than the stretches closer to Highway 59.
From the beach road. Connections run north from the coastal highway, which means anyone in a rental between the two towns can reach the network without a car. In summer traffic this is not just virtuous, it is faster.
For distance riders, the useful mental model is a spine with loops hanging off it: you can string together a flat ten to twenty-five miles without repeating much, and without meeting a hill anywhere.
Bike rental is available in both towns and through concessions in the park, and a trailer or tag-along is a normal request rather than an exotic one.