How many kilometers per day will you be doing on the tour?

How many kilometers per day are you currently doing?

If there isn't much of a difference, my only piece of advice would be to do a few rides with your bicycle loaded up with your panniers, and maybe a weekend tour to sort out what you need to carry and what you can leave behind.

If there is a big difference, start gradually increasing your daily kilometers and throw in days now and then where you ride with a load on the bicycle so you can gradually get used to the difference in handling.

Oh yes ... vary the terrian and weather conditions you train in. Not every day on a tour is going to be sunny, calm, and on flat roads. You'll likely hit days where the rain is coming down in sheets and you have to climb several mountain passes. Knowing how your bicycle handles in different weather conditions, knowing the right clothing to wear, and knowing that you've got the right gearing can go a long way to making your ride tolerable and safe.