Exercise! Diet alone will not do it. Once you get into ketosis, you can insure yourself against burning muscle by consuming less than 2000 calories a day but making sure that a significant amount of those calories come from fat (and most of the rest from protein!) The following advice is not medical advice. It's what has worked for me.

If your heart is safe and your joints are good (ie, doctor's approval) Start walking, and go until you are truly all done in (45 min. - 1 hour). If it takes you a week to recover, then take a week. And then do it again.

The trick with the type O in ketosis is to get to his or her limit, allow for adequate recovery, and then do it again until stamina builds up. If you are fifty pounds overweight and out of shape, then you'll have a lower setpoint than many people. But after several weeks (like 6 - 8) of intense walking sessions, you may be able to upgrade to something inherently more strenuous, such as jogging, martial arts, weight lifting, etc.

As an O, I do better for a longer session at one time, even if I have to pace myself, then to try to do several short sessions. One hour-long walk every four days is better than three twenty minute walks over five days. But the goal is to get to three long sessions per week without burning out (over training).

At present, I am in a resting loop and am doing three hour-long tae kwon do sessions per week. But in a heavy loop I do the taekwon do sessions and two sessions of weights with stair stepper per week. But it takes time to build up to this. And it also pays off to do "loops" or "cycles" to give your body a resting phase.