Originally Answered: Should I run twice a day on the 3 day diet for better weight loss results?
You SHOULD run twice a day, and you should NOT restrict your energy intake by dieting.
Energetic exercise like running is powered by sugar. Fat is physically unable to burn fast enough to 'fire' your entire bodyweight into the air as you need to do when running.
So you are correct to assume that you won't have sufficient energy. If you try to run, when your energy intake is insufficient, the rapid sugar-burning will lower your blood sugar level very quickly. Low blood-sugar causes weakness and hunger, poor co-ordination and lack of concentration (your brain is entirely powered by sugar too).
If it persists, you can get dizzy, experience confusion, feel faint and even become unconscious.
If you are diabetic, this loss of consciousness ('diabetic coma') can trigger reactions in your liver that will manufacture new sugar from other substances ('neo-glucogenesis'). In people who are not diabetic, blood-sugar levels cannot normally fall to the point where this process kicks in.
So if you are dieting, running even once a day is a bad idea. You can still be active, if you stick to activities that are gentle enough to continue indefinitely without feeling tired or hungry. This type of activity can be good for your body, but it works (and improves) much better with frequent bouts of more vigorous work.
So dropping the diet, and getting your running ability up, is a better option than trying to improve your physique through 'calorie' (in reality, energy) restriction.