The first thing that I would encourage you to do if you have not done so already, is to meet with a recruiter to do a qualification prescreen. This will allow you to discuss all of the expectations, as well as to take the ASVAB practice test to determine your strengths in the areas that the Air Force is looking for.
As far as the physical preparation, as a minimum I would start slow, pace yourself and set daily and weekly goals. At a minimum, you should be doing push-ups, sit-ups, and cardio activity. Try to work up to 2-3 miles a day walking, then running. It doesn't take a lot of effort, but if you work at it, within a month, you should be able to work up to several push-ups, sit-ups and a steady pace for a 1-2 mile run.
Don't forget to celebrate each new achievement! And even if you do not end up joining the Air Force, you will at least set yourself up to be healthier by instituting a good workout plan.