My dog loved people, now is very aggressive.?

I have a 1.25 year old Border Collie mix, 35 lbs. I got her from a shelter at age 2 months. I have read and generally tried to follow the Cesar Millan way - exercise, discipline, and affection. Due to lots of work and school over the last 6 months, the dog gets much less exercise and attention. She now is barking a lot more (out of boredom, I'd ume), but is also more aggressive toward people and dogs when we do walk. Will try to run up to people and bark loudly at them, enough that people are scared. I made the mistake of having her off leash recently, and she terrified an older couple. Is all of this a result of too little exercise, and no clear pack leader? What can I do since I don't have much time during the day?