How bad is it to go from little-to-no exercise to exercising everyday?!

Depends how you feel after the exercise. I would suspect that your tolerance for exercise initially will be pretty low anyway so you'll give in fairly quickly at the start, and over time your tolerance will increase, you'll be able to exercise for longer and harder, and your fitness will develop over time.

Obviously don't hurt yourself, if you're exercising and you're in pain then stop. Don't tear any muscles. If you're planning to do a big thing like climb Kilimanjaro you're best off speaking to an expert of some kind (GP, physical trainer) about what exercises you need to do and how hard to do them. A physical trainer should give you a programme to work to that you can manage.

It can be a little jarring for your body. Make sure you're stretching before and after each day, and don't go overboard. Though I think you're supposed to have days of rest in between workouts.

Its ok provided that you just increase your levels gradually.

GP for more years than I care to remember