The first year you do an equal balance of human and physical papers (no choice involved). For the second year you can select 3 human and only 1 physical if you want. For the third year, I think you might be able to select all human (I forget exactly; you might have to do one physical again), and of course can specialise in your dissertation as well.
When I was there, there was a better range of human modules to choose from and generally that side of the course felt better supported than the physical. Don't know if this will have changed at all.
As to work, the workload for geog. at cambridge is a lot less than some subjects there, in terms both of supervision requirements and lecture load. But it's still pretty intense work, of course, in those short eight-week terms. How much work it actually demands in terms of hours per week varies so much by person it's not really worth making generalisations.