Start with a 3 day trip, just roll out of the garage, go somewhere, have a day off, and then roll back. It should be about 25 miles. You need a few warmup rides to get used to long days on a loaded bike. You can even load up your bike, and go for a long ride weekends, making the bike a few pounds heavier each time while you also slowly increase distance.
The heat will make the seem pedalling harder, so you want to be ready.