A.V.
I totally understand the need to get out and not feel panicked if you are not back in 2 hours. It is good to know that your baby would take the formula if she absolutely had to. I agree with the previous poster about gradually diluting the breastmilk with formula (remember to make the formula liquid first with water, dont just put the powder into the breastmilk).
I personally believe that breastfeeding is the best for our babies, and even though I sometimes felt uncomfortable breastfeeding in public at first, now I refuse to let anyone make me feel weird about nourishing my child. I know it's a personal decision, but I know that the more of us who do it with no shame, the less weird it will seem to others!
Do what's best for you. Just keep in mind that it's for such a short time that you have this opportunity. All the best!