Something punishing is extremely physically difficult to do. On a very hot, humid day, even a 5K is a punishing race to run.
Anything that exhausts you because it's so demanding can be described with the adjective punishing. A punishing hike leaves you gasping and applying bandages to your blisters. A punishing argument with your family is more mentally arduous. Sometimes, punishing things weaken or debilitate: "The lack of snow had a punishing result for the ski industry." Rarely, punishing also means "leading to punishment."