Frankly it pains me every time I see someone's wife / girlfriend struggling to keep up on a badly maintained $400 bike while their other half is on a shiny $4000 bike. In my view this is unhealthy for your relationship, shows the other half doesn't help maintain their bike as well as making one half look like a tightwad.

Why not loosen the purse strings and get the GF the same bike as you, or at least one which costs within a factor of 2 of yours? I'm sure she'll appreciate a nice gift that gets you both out exercising together. If you want to push the boat out, or if you're of very different abilities a tandem is nice as it will stop you from hairing off up the first hill. Believe me you can get a very good workout over a short distance on a tandem if you want to.

A tandem and a bike is even better, but you don't need to buy both at once.