What he [Trailrunnergary] said; see above, I will not waste space repeating it.
I have a few additions that were important for me and which, in observing friends seemed to be missing.
I found that I was best when I did some of my long runs (at least most of a run) at something like marathon race pace. Not all of the long runs but at least a few (and hence, 'b' works and 'a' does not because you cannot get enough of both).
A comment was made about teaching the body to burn fat. However, another thing that you need the body to do is to respond to running out of glycogen so that it can respond by increasing storage. Running slow or even moderate pace for long runs will not get you to the point of running your glycogen stores down towards zero. When you do you also probably get the body to deal with HAVING to burn fat, not being slow enough so that it gets to simply burn fat.
Finally, without getting into the region where you stress the body in the fuel domain you will not stress the mind with the reaction to how it feels. Hitting it for the first time in a key objective likely leaves you in poor position to deal with it. This is especially the case for glycogen exhaustion because when you run low your thinking can get slowed and psychologically it gets hard to deal with the situation in that deprived state. (This is also a reason to slightly negative split because you will be passing people steadily the last 10k and it helps the mental state and I always found it easier to push myself as a result.)
While the following does not work for everyone it might be useful for some. Since I was a good cyclist (raced/ won a high % of Cat III races), I did some long rides at good pace as well. This pushed my body/mind without the wear and tear on the legs etc.