Because it’s not a good workout unless your body starts to fail.

An interesting article was recently written describing the vision loss seen in ultramarathoners.

Any footrace longer than 26.2 miles. Common race lengths are 50K to 100K runs.

The condition is called Ultramarathon Associated Vision Loss or UAVI. It’s described as a self-limited, painless clouding of the vision due to corneal edema. One theory is that stress to the cornea (hypoxia/altitude, cold, dehydration, debris, wind, etc.) can lead to a buildup of corneal lactate, which can act as an osmolyte in the cornea, drawing more fluid in.

A suggested treatment is Muro 128 to draw out the excess fluid. Here is a picture of a guy with corneal edema after running 100 miles. (via)