We all want to drop into that "zone" state. This is where time flies and we are intently focused on the activity at hand.
Learning meditation is like learning to ride a bicycle. While sitting on the cushion, we want to coast peacefully, without distraction. It's not too different from bicycling. Imagine you are on the interstate and there is no one else on the road. There's a long stretch of highway for miles and the temperature is perfect. The ground is level so you don't need to pedal so much to keep your pace.
These are the conditions that lead to coasting. But think of all the components that must come together so that you are out there and you can enjoy your ride. You must have the endurance to travel this far out, the bike must work in harmony, and you must be familiar with the environment.
This is the same with meditation. Coasting is to bicycling as the theta state is to meditation. After some time with your practice, you should have endurance to sit comfortably in your posture. Also, the mind is controlled and free from distractions. Lastly you are proficient with breathing. These are the criteria that will allow you to coast in the meditation bicycle.