Even more often you will make them from scratch. Do major details orientate yourself on, then get to smaller details. It goes somehow like that. No tool nor method is forbidden with reliefs!
You may want to cut parts of already done reliefs and place them on others, rotate them, play with their levels, thus creating new reliefs without picking a brush. This will not be enough though, so, then you will pick a brush and do more editing.
@LRV, ha-ha, creating landscape is a lot of pain for me as well... But if you take the right tool and learn some tricks, it should be very possible for you then.