I increasingly like light and fast tours, and I am just back from a short one in the Ardennes and Eiffel, with just two front panniers on the rear rack, plus a tent on top (and a bar bag). With the two of you, it should be a bit easier still, since you can share things like tools, and because e.g. a two person tent is lighter than two solo tents. Even so, you need to reduce luggage weight as much as you can (10 kg for the gf, and maybe 12 for you, i.e. she with just the small panniers or a Carradice Camper Longflap saddle bag, and you with the tent as well). This only works if you can keep the volume of the sleeping kit low, as with two light down sleeping bags and something like Exped Synmat Ul mattresses.
If the bike has fittings for a rack, go for a Tubus Fly or Vega depending on your frame. As for the bike itself, make sure you lower the gearing. Modern Shimano rear derailleurs can handle 30 t cassettes, and you will be happy that you fitted one. Similarly, have a look at your chainset to see if there are easy ways to lower the gearing a bit. Raise the handlebars a bit if you can. Fit the widest slightly sturdier touring tyres that you can, like the Panaracer Pasela. Have a serious look at the rear wheels, particularly of the heavier rider (you?). A traditional handbuilt wheel with a stiff rim and 36 spokes is much more reliable than many fancy and fashionable road wheels.
Finally, 100 miles a day is possible, but only for very well trained riders who are prepared to ride long hours.