Watts, like horsepower, measure your instantaneous power. They don't measure work over time. So you did not do 1/2 horsepower of work over 2 hours, you may have maintained 1/2 horsepower of output over 2 hours (which is a hell of a lot). That would be about 3000 kcal of work, which you can convert to watt-hours, which would be about 3500 w-h.