It's utterly obvious that you will be tired after you workout; but did you know you shouldn't be. If you find yourself being tired, chances are you are doing everything wrong, and are overexerting in areas that don't need it. Cardio workouts are not supposed to make you weak and tired; but rather help you lose weight, and keep in shape. So logically speaking, if you find yourself tired always, you are not in shape, and the cardio workout you are doing needs the following improvements;

Heart-rate- it just needs to be up, it doesn't need to be beating faster than the earth spins. Many people make this mistake of going hard at first; but then they become burnt out fast. This is the easiest way to get tired, over exert yourself; and your heart rate goes so hard, that you can't pump the oxygen fast enough, and then your brain notices, and you become tired all around. Then you are burnt out in other things to, because you just lost oxygen to your brain very rapidly- and the only way to replace this, is to; guess what sleep.

Water- working hard for a long time, sweating a lot. You lose a lot of water weight, its not actually weight loss, its just water loss. Exerting yourself to lose a lot of water, and not replacing it, makes your body exhausted, and your mind as well; then you will always find yourself being tired... You need to replace what you lose, water wise; always hydrate your body, many people get strokes and so forth, just because they don't hydrate themselves, then they are mentally scarred for life.

Sleep- well obviously you need to have adequate rest in the meantime- 8 hours is recommended, if you have work and exercise and not much sleep; over time this can be a harmful combination, and make you so ill that you have to take a few weeks rest for recuperation. Your mind and body will not be overly tired at first, but over some time you will find yourself just drained. Fix these 3 points, and you will find energy coming a lot more afterwords; and fatigue reduced drastically.