I think if you've been exercising, it would be fine to keep going during Phase 1. I managed to keep up with my running, yoga, and elliptical workouts during Phase 1. I didn't increase my intensity at all, but I kept it up.
As far as what to eat...I don't eat before I exercise. It gets all knotted up in my belly if I do. If I were you, I'd eat a good, solid dinner the night before with a bunch of veggies and some beans, if you're used to eating beans. Then when I got back, I'd have a protein shake.