I wrote all your posts, because now in Qatar are 12:30, lunch time. I'm surprise to see a lot of posts after my last of yesternight. So, I try to answer to many general questions, of course under my point of view.
a) May be that I had some mistake in calculation, because I did everything very quickly. In any case, if 2:32 is 15.2 every 100m, and 3:17 is 19.7 every 100m, the difference (I use this system, please don't come with mathematic...) is 4.5 every km, that is 33% of the time of the fastest pace. So, if 2:32 is 100%, 3:17 is 67%. Of course, if you consider the two full times, we have 77% of effort (152.0 divided 197.0). In any case, this is a fact, you can decide the percentage according to your personal rules.
When I speak about 85% of the speed, I speak about the long run of 25 km, that Shaheen carries out at 3:10 average or faster, using a progressive way (some time last 5 km in 14:30 about).
b) I'm sure that the ability in increasing the ability (forgive my repetition) in using fatty acids is very important, but one thing is to work for increasing this ability for very long run, another thing to increase this ability at fast marathon pace. I adviced one of the Italian winning twice World Championships in 100 km, Mario Fattore, and can say that training for ultramarathons is very simple, because you use ONLY FATTY ACIDS, forget carbohydrates. So, your problems regards other situations : to preserve good fitness for so long time (7-10 hours), to build tendons, ligaments and muscles for that type of duration, to build your mind. But no problems about the source of energy. Also if can seem strange, we need very less training for running 100km that for running a Marathon. For 100km, you have to develop your MAX AEROBIC LIPIDIC POWER, using types of fatty acids able to give more energy in the time unit, and the system of training starts from very slow speeds, at first developing the DURATION (one session every 2 weeks of 60-70 km, and some full marathon like.... speed), after the pace inside the same duration. So, always you use fatty acids, but increasing speed you become able to select the bests in order to develop your Aerobic Power.
Instead, when you want to increase your marathon at high level, YOU START FROM SPEED AT MARATHON PACE OR SIMILAR, and the first step is to use ALL YOUR CARBOHYDRATES in order to run still some minute in total depletion of sugar, for obliging muscular fibres in using THE MOST POWERFUL SOURCES OF FATTY ACIDS in order to maintain a similar speed. For example, if an athlete able running HM in 65.0 wants to prepare a marathon for 2:15 (really not difficult), he can run without big problems 28km at 3:15 pace (1:31), but after this goes to finish his reserves of glycogen, and his pace goes down dramatically. If he is able to run yet 3 km at 3:25 / 3:35 / 3:45, THESE LAST 3 KM ARE THE REAL TRAINING, because his fibres have to go to search some other hidden tank, under necessity. The next time, the same athlete becomes able running at 3:15 not 1:31, but 1:41 (for 31 km), so his long fast run must be extended to 34 km with the last 3 without glycogen.
So, in the first case we start with an empty glass that we go to fill with fatty acids, in the second case we start from a glass full of glycogen that we go to empty step by step, in order to fill the remaining part with qualified fatty acids.
When I speak about FAST LONG RUN, don't think that ALWAYS long run must be fast. If I go to prepare a Marathon, I use one specific long run every week, alternating two different ways :
a) DURATION - I don't have particular care about the pace. I start from 1:30, and in short time I move to 1:45 - 2:00 - 2:15 - 2:30. This training is at 70% of your HM pace (for ex., if you have a PB in HM of 70.20 that is a pace of 3:20 per km, and is the first time that you want to prepare a marathon, your pace for DURATION can start from 4:20. At the beginning of your training (may be 5 months before your Marathon), once a week you go for DURATION, in order to reach the ability in lasting the full marathon time, in short time, because your long run is very easy.
b) When you are able running, for example, 2:30, you can start to qualify the DURATION using a progression for the last 15:00, after 30:00, and so and so.
c) But, when you are able running 2:30 of DURATION, you start also your training of DISTANCE. Distance is a precise distance (forgive me again), that you go to run at even pace, about 90/95% of your HM pace (and this has to become your Marathon pace). So,. in the previous case, 95% of 3:20 is 3:30. You can start with 24km at 3:30, and the progression is TO EXTEND THE SAME PACE, running 26 and then 28 and so and so.
d) When you have both DURATION and DISTANCE in your program, the system is TO QUALIFY THE SPEED OF THE DURATION, and TO EXTEND THE DURATION OF THE PACE OF THE DISTANCE. You can alternate these 2 long runs, one in one week, the second in the next week. At the end, you become able to build a funnel, where your DURATION is 2:30 at 3:40, and your DISTANCE are 36 or 38 km at 3:30. When you are able to do this, be sure that you can run your full marathon at 3:30 finishing faster the last 10k, and, if you go for a HM one month before your Marathon (of course we can use, in every week, another specific workout of extension : so, no more than 2 workouts per week), your PB can move from 65 to 64 without big problems.
All the other days are for GENERAL VOLUME (many km slow or following your sensation, in any case easy) or for RECRUITMENT OF THE HIGHER PERCENTAGE OF FIBRES (using very short sprints uphill).
e) Regarding the fact of very long run, I want to remind you that, for every type of event and for every specialism, we have 2 different kinds of typologies : the FAST and the RESISTANT. This fact depends from their morphology and their physiology, and in part on their psychology too.
For example, how many strong athletes of 200m can be better on 300m, but are not able to run 400m very well ? And, if there was the official distance of 600m, are you sure that could be Borzakovskiy of Bungei the winner, or not, for example, the best Mutua ? Don't forget that the distances of official T & F are conventional, and every athlete has to adapt his training to them, but probably could have a better distance for his attitude that doesn't exist (remember when, in the past, you have in US, during indoor activity, distance like 500y - 600y - 1000y, and go to see who were the record holders).
For example, if you are an athlete able running 200m in 22.0 and 300m in 34.0, you are RESISTANT ; if you run 21.5 and 35.0, you are FAST. In the second case, forget that you can become a good 400m runner. In the first case. move to that distance soon, and in 3-4 years you become able to run full 400m at the same pace of your 300m before (in this case, 46.5) or very close.
This for explaining that there are runners that NEVER IN THEIR LIFE CAN BECOME BETTER USING LONG RUN. Why ? Because their internal anathomy (muscles and organs) is the anathomy of a sprinter, with more fast fibres and different nervous ability, and a different hormonal system, and different psychology, etc.
Also among animals there is the same situation : the lyon is a sprinter, and, if is not able to reach the gazelle after 400m, goes to rest awaiting the next victim...
So, don't generalize what I write. My phylosophy in training is to find the best solution for obtaining the best individual results, following the best personal attitudes. But, in the amateur field, everybody wants to do the event that he prefers, not the best for himself. And I suppose that this is right. So, if I see Alan Webb running one mile, and I meet him on an elevator during a meeting with a T-Shirt with written "MILE RUNNER", I tell him "For you is better to write 3 MILES RUNNER" (may be that he remembers this fact), because I think that 12:52 in 5k is better than 3:32 in 1500, and probably in the future 26:40 in 10k can be better than 12:52, and also in the last lap of 10k he can be the faster, in the last lap of 5k can win a medal, in the last 300m of 1500 of sure he has 5 athletes ahead him. So, my advice for TOP ATHLETES regards the way for reaching their best results, not what they prefer of like. But for all the other, I suppose that you must do what you like.