Whether you're looking to start exercising for the first time, or just trying to get into it after a long break, you can use HIIT to do it (although initially it might be more accurate to call it Low or Moderate Interval Training rather than High). This is the approach that I use to get back up to speed after long periods of inactivity.
Day One: Begin with day one of Hiit It. Perform only one cycle through the exercises, and don’t go at your maximum intensity. You can do just one or two reps of each exercise, if that feels like enough. Take breaks if needed. Substitute easier exercises as needed (e.g. hold forearm plank instead of doing commandos)
Day Two: If you feel good the next day, you can try two cycles through- you don’t have to do them back to back unless you want to- you could do one cycle in the morning and one in the evening, and stay at a comfortable intensity for your current fitness level.
Day Three: If you feel good, workout a little longer and push a little harder (trying to do more reps in the same period of time than before, while maintaining good form throughout the movement), or if you're too sore and tired, go at a low intensity one or two cycles through. The important thing is to keep going, each day, even if that means taking a step back, just don't stop completely.
Continue building up at a comfortable yet challenging pace until you're at 30 minutes of high intensity 3-5x a week, and are ready to work your way through the full 12 week program starting again at Day One.
This is a good approach for those of you working without a guide as well. Pick a few exercises (squats, plank, mountain climbers, etc) and try to do a few more each day (or every other day) than you did before (if you did 5 squats last time, try for 6 this time), until you're up to a full 30 minute workout. You can swap or add new exercises in as you wish.
This approach is one you can grow with. Trying to jump into a rigorous exercise routine (either because you're excited or because you have in your mind what you *used* to be able to do with ease), is overly stressful to the body and more likely to lead to fatigue, grueling soreness (ask me about my first enthusiastic legs day), disappointment, and abandonment. It can be difficult to hold ourselves to doing just a few minutes of exercise the first day, and building up, but there's such a better chance of long term commitment and success. Far better to start slow and adjust up than start too fast and really throw yourself off.
Walking can be a great way to build up your fitness (especially if your exercise break was due to injury- be very careful about what exercises you do, your form, and how it works with your body particulars when beginning a fitness regime after injury). Walking is great on your off days as well (active rest days)- it's really important to keep moving to some degree every day (tailoring the intensity to your needs- lowering intensity when you're sick, menstruating etc), for general fitness as well as to stave off those sneaky periods of inactivity. We fall out of fitness one day at a time, and we grow into it one day at a time as well.