The answer is very easy :
a) if you prepare marathon with the modern training, and of course you are a "fast" type runner, coming from good times in 10000m, you can have a top in shorter distances BEFORE the marathon (for example, 6 weeks) looking at HM, and AFTER the marathon (for example, 2 months) looking at 10000m.
This because the modern system is : during FUNDAMENTAL PERIOD to increase the AEROBIC POWER, and during the SPECIAL PERIOD to extend the ability in lasting at high percentage of the Aerobic Power. After the marathon, we have a short recovery (3 weeks), reducing mileage and putting in training some special exercise for recovering the qualities that long run goes to consume (elasticity, reactivity, agility). During a so short period, the athlete doesn't lose too much of his endurance. So, immediately after this recover, he can start training of specific speed endurance for 10000m, with the possibility to enhance the specific volume.
If, for example, when you prepare 10000m coming from the speed, you can run 10 times 1000m a little faster than the pace of 10000m (for ex., for an athlete having 28'20" that is 2'50" per km, 10x1000m in 2'47" with 1'30" recovery), using the endurance you had after marathon, you can run 15x1000 at the same pace, and this means an IMPROVEMENT IN SPECIFIC ENDURANCE.
Of course, when you are able running 15 times 1000m in 2'47", you are able running 10x1000 in 2'43" / 2'44".
So, the effect of having more endurance is TO ENHANCE YOUR SPECIFIC SPEED.
b) If, instead, some athlete thinks better to push in direction of speed, with low mileage (and especially
not using long fast runs in any session), not only he runs SLOWER 10000m, but NEVER can become competitive in a full marathon in a period of 3-4 months.
But, practically, we have to face a problem : if an athlete wants to stay at his top for two seasonal peaks (marathon in Spring and in Autumn), the mental concentration in training must be so high that it's very difficult to preserve for a full season nervous energies, enough for being competitive everywhere.
That's the reason because it's not common to find top runners able, in the same year, to run at their best both marathon and 10000m ; however, under methodological and physiological point of view, this is possible, coming from marathon to 10000m, not from 10000m to marathon.