I have had the same problem as the OP.
Prioritize, first: is she a good enough non-cycling friend that you would not want to lose her as a friend? IME, it can be difficult due to time constraints to find a good friend. If you already have a good relationship, find some other way to achieve your goal(s) and preserve your friendship.
Increase your intensity. Don't just stuff a few weights in your panniers, get a rack that can handle upwards of 40lbs. Increase your load so much that you struggle to keep up with her as she sets the pace. If 40lbs is not enough, get yourself a $100 100lb capacity Wal-mart bike trailer and haul water bladders (at 8lbs/gallon) or something else.
IMO, you want to train at a higher intensity level than you expect to tour at, anyways. I have not toured, but I haul a lot of stuff back & forth to work. It is not a sprint, but more a marathon. I save my top speed for situations where speed will be of use. Otherwise, I choose a pace that I can maintain for a long time and not exhaust me.
If she wonders about the increased load, tell her you are training for a tour and that you expect to ride at a similar pace, but under greater load. Heck, she might be holding back because she assumes you are still a newb or might work to increase her pace to be of greater help. Keep it positive.
One added benefit it to see how both you and your bike (& maybe trailer) perform under load.
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Oh, my "problem*" is my wife.
We work hard to spend extra time together, especially exercise time. When we walk, I strap on a rucksack and push the kids in the jog stroller. When we bike, I carry almost all the gear in my rack basket and tow the kids in the trailer. This allows us both productive time together and we both get something out of it. When the gear & kids are not enough, I carry more water, "just in case," to up the intensity.
* Obviously not a problem, but a circumstance of my exercise load requirements being greater than those of my wife.