Originally Posted by Machka
In addition to increasing your mileage on 1-2 days per week, I would strongly suggest riding in a wide variety of terrain, in a wide variety of weather conditions, and with your loaded panniers. In fact, I would suggest going on a weekend tour about a month before your big tour to test out all the equipment etc. ... cycle out to a campground somewhere 50-75 miles away on Saturday and back again on Sunday. That way you will have a better idea of what you need to bring with you, and how well your setup works.
All the best!
I start with lightly loaded panniers about 6 to 8 weeks before departure. Add weight to them as you get closer to your departure date. Since loading panniers with all of your touring stuff is kind of a pain, I'd suggest using rice. You can find it in any grocery store and it comes in 1, 2 or 5 lb bags for around a $1 per pound (or less). It's nice and heavy without being too bulky and it is far less abrasive than carrying sand bags. I double bag each rice bag with a plastic grocery bag to keep them dry and intact. At the end of the training, you can either eat the rice (and trust me 40 lbs of rice lasts a very long time) or donate it to a food pantry.
I've ridden many miles back and forth to work carrying lots of rice. People give you some odd looks when you tell them what you are carrying but when I'm out on the road the training really helps.