A race in which the animal being raced finishes in a state of physical exhaustion.
present participle of gruel
characterized by effort to the point of exhaustion; especially physical effort
"worked their arduous way up the mining valley"
"a grueling campaign"
"hard labor"
"heavy work"
"heavy going"
"spent many laborious hours on the project"
"set a punishing pace"
So difficult or taxing as to make one exhausted; backbreaking.