I must be the exception then. I put on 2000-ish miles on the trails every year, strength train twice a week and I'm no where near an ectomorph (albeit not any where near an endomorph either).
Genetics will play a large role in this. The level of performance you´re looking for on each end of the spectrum will as well.
Would you be more competitive in a local half marathon or a local powerlifting meet? Knowing nothing else beyond what you´ve told me about your training, if I had to bet my life savings I´d guess the former.
If you were able to look at the entire population, I guarantee you running mileage would not correlate with lean body mass. Look at collegiate 5k runners (who do tons of mileage for the sake of aerobic base building) and then look at collegiate weightlifters.There are some very apparent morphological differences.
This is an unavoidable reality of physiology.
Training for anything comes down to stimulus, recovery, adaptation. The adaptation that will occur is specific to the demand imposed. When you go out and do your long day in a typical mid to long distance running program, you are not telling your body: ¨Get bigger, get stronger.¨
Concurrent training for maximal strength and aerobic work comes down to making smart compromises. The interference effect is something pretty much everyone in S+C agrees exists to some degree.
ned S and E training programs 5 days · week−1; the control (C) group did not train. After 8 weeks, the E and S+E groups had similar gains in endurance running performance, the S group had no change, while the C group showed a decline. No strength gains were noted in the C or E groups, but...
journals.lww.com
This isn´t doom and gloom for average folks. You just have to learn how to program both things in such a way that they interfere minimally with one another, and the one that needs to be prioritized gets prioritized. Alex Viada (despite his dubious mile time claims) has a good book on this.
It also depends on what level of performance we´re talking about. Squatting 255 is totally totally doable on a high mileage running program for most of us mortals. Squatting 405 not so much.
Which is more important would depend on a specific athlete´s situation. If a guy is trying to stop being an ectomorph, his squat is more important than his 10k time.