How can alcohol affect your fitness?

It depends whether you mean a temporary effect, such as during a time while you are drinking, or whether you mean taking excess alcohol as a life-style. Obviously alcohol slows you down, mentally and physically.

If you habitually drink to excess all your functions will slow. More sinister is the effect on the liver. This is what kills people who are inveterate drinkers: cirrhosis of the liver is something no-one wants to suffer.