I physically train before I hike long distances, but it is not strictly necessary unless you desire ambitious mileage. As long as you are generally fit and in good health. Either way it takes some time to adjust to walking every single day once you are on the trail.
Skills and experience are as important a part of planning as reading guides. Try hanging a bear bag or digging a cathole before you actually need to on the trail. Considering taking a Leave No Trace class if these skills are unfamiliar.
It is also important to figure out your hiking style. There are many ways to hike and it is not helpful to plan an endurance hike if once you are on the trail you find that you like exploring towns. A longer gear shakedown trek can help you to figure out your hiking style.