Yellow, hang in there. You can do it. Those long back to back days are supposed to suck just a little. You ultra runners are incredible. A friend of mine just did the Leadville 100 (although he had to drop out at mile 76 because of hypothermia). It's incredible what the mind can do to overcome the fatigue of the body. Mind over matter. You can run those back to backs and be ok- even in the long weeks. During the peak of my training for IM, every weekend was back to back long days (and I really believe the fatigue training paid off race day). I KNOW running only is much tougher on the body, but you're one tough chickie, and you can do it.
If you bail and "only" do the 30K (HA- like that's easy ), you know you'll be fine. Either way, you're a rock star in my book!