Maybe you are too weak to go for such a tough work. If you lack of nutrition or your blood pressure are too low or too high, you will get blurry vision after running because running is an aerobic sport. People who lack of nutrition may find themselves lack of oxygen and dehydration. If you always have this problem when running, you should go to see a doctor to have this checked. And just take the doctor's advice.
After running for a long time, the blood going to your legs and not other parts of your body. Maybe this will cause your feel vision blur. I also often feel vision blur while in intensive exercise. But i just feel vision blur during the exercise not after the exercise. That just only last for a few seconds and it will recover clear vision again. And i think maybe it is caused by body fatigues. I don't think it is a big problems.