So how hard do you have to exercise to get benefit ?

How hard should you exercise?

There is a simple test called the sing/ talk test.  If you can sing a song you are probably exercising at light intensity. Moderate intensity is the best level of exercise and the rule of thumb is if you can comfortably carry on a brief conversation but be working enough that you would not be able to sing a verse while exercising, you are probably at moderate intensity.