"hard workout" is correct and usable in written English.
You can use it when referring to a strenuous physical activity. For example, "I'm going to attempt a hard workout this afternoon."
Completing a hard workout, then, is just the first step.
I approached this race as a hard workout.
A hard workout for one person can be ridiculously easy for another.
Nothing combats feeling depressed or anxious like a good hard workout.
So now I've tapered back to a really good workout in the morning and a really hard workout at night.
Right now, he intends to gallop Mine That Bird into the Preakness rather than give him a hard workout.
From the perspective of an athlete, few things top the virtuous satisfaction that comes from a hard workout.
"If it's so hot you can't get a hard workout in, it defeats the purpose," said Dr. Casa, who tests his theories trail-running in the heat.
With the race on the line, she has described the waning days of the campaign as being "like the end of a hard workout".
If your metabolic rate increases slightly, why would you feel warmer as much as seven hours after a long, hard workout?
