Question

If you're aerobically fit you can

Answer 1

When you're aerobically fit, your abilities to transport and utilize oxygen, measured by: the maximal amount of oxygen (VO2 max) which naturally occurs in the body while doing intense and low or exercises. More blood is pumped through out the body when being aerobically fit. More oxygen is sent to your brain, leading to better brain power.

Answer 2

If you're aerobically fit you can sustain a regular routine of exercise for 30-60 minutes with minimal fatigue.
If you are fit, it means that your body can handle a lot of exercise with minimal fatigue, because it is prepared due to constant exercise. It doesn't mean you can lift very heavy weights - you need a lot of body mass, muscles, and strength for that. It doesn't mean you can run a marathon and not feel tired at all - you will always feel at least a bit tired no matter how prepared you are. If you quit your normal exercise routine, there will be some consequences at least.