How do you begin an exercise regime after a long period of inactivity?

I’m hoping someone can advise me on how to start off a good fitness regime after a long period of inactivity. I’m a 22 year old woman, about 5’4″ in height and currently 113lbs. When I was in my senior year of high school, I was 125lbs, and fit as a fiddle aside from suffering from anxiety throughout much of my teen years. The anxiety attacks made me lead quite a sedentary life after high school, and I’ve also gained a rather perplexing stomach ailment in the past two years that caused me to lose over 10lbs, and suffer from a lot of bloating and abdominal distension. My doctor seems to think this is IBS (irritable bowel syndrome), and I’ve been advised to change my diet and exercise more. I’ve cut dairy and red meat (my biggest trigger foods for gastric upset) and have started trying to exercise more, but I’m so out of shape that I overexert myself with the simplest things when it comes to cardio. I’m out of breath after a short sprint, and running, jogging, biking, and rollerblading seem to make me feel like I’m going to fall over and pass out if I try to attempt even half a mile. It’s like I’m fit enough to move fast like I used to, but once I do it, my heart starts pounding and I’m gasping for breath. Walking is about the best I can do comfortably, and a I’m worn out after a mile, with my legs really aching for the next few days. It’s very discouraging and making me not want to exercise, for fear that I’m going to harm myself if I’m not careful enough, or just wanting to avoid feeling horribly uncomfortable for the days after a workout.

My fitness goals are to tone my muscles, gain some healthy weight back (and maintain it), and be able to run freely again without exhausting so quickly, and eventually gain the stamina to go farther distances. What’s a good way to go about getting back into a fitness regime?