Pain is the microscopic tearing of muscle tissue through strenuous exercise. Where you benefit is during rest and recovery, where your muscles repair themselves in the form of scar tissue that mends the tears. The result is bigger, stronger, harder muscles that make women swoon, and where you are the complete envy of modern man, to be forever known as "that bike guy". (as if that can be achieved wearing lycra)
But to the original question, I'll relate to you what my cardiologist told me. "Any time you elevate your heart rate to anything other than couch potatoe status, you're doing good for yourself." "How much, depends on you!.......Too much, depends on god!"