26" LHT would likely be the easy answer. I dunno if it'd be all that quick compared to a speedy road bike but it would be versatile... the long haul trucker has a strong following so hopefully someone will chime in about their experiences.
I have a red norco ATB frame from the late 80s that is pretty tiny and was gonna build it up for my mother who is about your size... it would make a great touring bike, it has all the braze-ons and long chainstays. Even high end vintage mountain bikes seem to be on the cheap side, and they usually come with all the gearing for touring you'd need. A drop bar conversion is pretty easy for most of them.