In most hunter-gatherer societies now, men hunt the megafauna, and women hunt small game and gather vegetable foods. The women do contribute more calories to the family diet. While 10-15 hours a week was spent gathering food, some foods require a lot of preparation, like nuts or plant pith ( which actually is a staple in some societies). So yes, women may have worked more than that. Compare it to the work of an 18th century housewife, working at a textile factory and raising a family, and it's still a lot simpler. As far as women inventing agriculture, most recent theories I've seen call it an accidental discovery. Women may have contributed more due to their portion of the gathering work (grains are domesticated much earlier than cattle), but I haven't seen any arguments on the subject.