You are correct that duration is the most important aerobic training stimulus. However, if your goal is to run faster and cover more ground in the same time then you will need to do at least some of your base training at higher intensities and faster paces. At some point long and slow becomes a less valuable stimulus than shorter and faster.
If you’ve been training like this for many months then I think you will get more benefit out of a slightly faster 30km than a slower 30km. If you struggle to make 30km then keep them slow.