You can run a marathon on three days a week of training. I just did it. I've read that you can do as little as one weekly long run and two 30 minute runs during the week (I typically ran more time/distance during the week).
If you're running 8 minute miles and that's what you are comfortable with, go ahead. As you get to longer distances, you may find that you want/need to slow down, but you may not.
Also, a higher cadence (90 per minute rather than 80) is generally viewed as more efficient and may help you go longer distances, but changing from being comfortable at 80 to being comfortable at 90 may take some effort/time. Basically you have to work on shortening your stride while increasing your cadence.
I also don't think you "need" to do running at specific speeds. I think those things are built in to a lot of training schedules because people have specific time goals. Or feel that it's too monotonous to run the same speed all the time. If your goal is 4 hours and you're comfortably running 8 minute miles now for distances over 10 miles, you may not need to add things like speed work to get there.
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