Focus turns time into depth.

Hours by themselves produce quantity. Hours with sustained attention produce something different in kind: understanding, craft, the kind of work that could not have been made by anyone passing through quickly. Focus is the mechanism that converts one into the other.

Stop counting how long you worked and start noticing how deep you got. Ask, at the end of a session, whether you went anywhere you could not have reached in ten minutes. If not, the next improvement is continuity rather than effort.