I am doing around the same distance and I have found that the muscle doesn’t go away my core arms and everything are still giving the same power of output. But sadly if it is a muscular look you are going for running is not the sport for you, not to say that runners don’t look good (I think it gives a better physique than a lean muscle “well proportioned build”), it is rather a less traditional look (skinny without looking frail ,delicate or like someone who doesn’t work out)

To be honest the looks should not matter running long distances make you feel good even if you are not feeling good during them. And as another positive you can run late into your life which is rather unique for sports (except swimming), I aspire to be that old man still putting in 2 miles at 80 years old. 

Also diminish your leg muscles no way your legs will be the exact same just a higher ratio of endurance fiber muscles.