“Should I work out as hard as possible if I want to get in shape?”

The answer to this question isn’t simple. Not only does it depend on your current level of fitness, but also on your fitness goals. The simple answer is no, but hard workouts should be done as often as your body is ABLE to after fully recovering from the previous strenuous workout. World-class athletes do not not train hard during every workout. Bear in mind that they work out as many as a dozen times per week, which the average person cannot, and many of these workouts are actually ‘recovery workouts’ which are not strenuous to the body. Those who are just getting into fitness will not be able to handle as much of a training load as those with extensive fitness backgrounds, but can work up to multiple tough training sessions relatively quickly.
So start slow, and work up to more intense sessions as your body allows you to.