Lyric discussion by bcessna 

Cover art for The Luckiest Man lyrics by Wood Brothers, The

Could be a reference to Lou Gehrig? Google luckiest man you'll see what I mean.

Lou Gehrig's Disease or ALS has no known cause = mysterious forces.

Up against too many horses = Eventually most people with ALS are not able to stand or walk, get in or out of bed on their own, use their hands and arms, or communicate. In later stages of the disease, individuals have difficulty breathing as the muscles of the respiratory system weaken.

Running is useless, and fighting is foolish = Although respiratory support can ease problems with breathing and prolong survival, it does not affect the progression of ALS. Most people with ALS die from respiratory failure, usually within three to five years from the onset of symptoms.

Your not gonna win = There is no cure for ALS.