gorshkov's answer is right on. Sounds to me like you may already be in touring shape, least ways for flat land and short riding days.

If you've got access to a hilly training route, set aside an hour a day and run it as fast as possible for a couple of weeks. No hills? Load the bike with 40 pounds and ride as fast as possible for an hour a day for a couple of weeks. Gear so that you are spinning the pedals at least 70 rpm. Either way should have you in shape to ride loaded at a reasonable pace for 4-6 hours/day. This oughta get you 40-60 miles down the road, which is about average touring pace on a loaded bicycle. You'll have plenty of time for short and long breaks, and your conditioning will improve, to a point, each day you ride.

Touring is 90% mental and 10% physical.