Go fixed if you have hills .... you will be riding faster than your GF, more control on the downhills - unless you release your feet from the pedals - and your climbing will improve significantly.
... charge the hill and wait for her at the top of it. That will make you stronger = the uphill sprinting + the resting time. If you are not tired, go downhill and charge back again, but slower. If she is still climbing it during your second attack, do it a third time going along with her. THAT is the way I do it while riding with Ms Huhenio.
Endurance riding with the SO ... that is another story.
