I don't think there's an easy answer because whilst some people might say "the best exercise for running is running" - if you can make the time, I believe that x-training and core/strength training can definitely complement your running, possibly lower your risk of injury, and improve overall fitness. Also don't completely discount the benefits of treadmill running. The biomechanics are not exactly the same but it's still running and you're still getting your heart rate up.
I think discipline comes into it as well. When I was running 6 times a week I tried to kid myself I could get into a regular core/strength routine at home. It just didn't happen! Now I go to the gym as part of my weekly routine and do specific sessions on different days, either concentrating on cardio or strength exercises. I rejoined the gym a few months ago, mainly for injury rehab, but I'll certainly keep up the habit when I'm back running.
So time is an important consideration. Will 4x running + 2x gym sessions make you a faster runner than 6x running? Probably not, but it may make you fitter in other respects. Alternatively if you can keep up the same amount of running training and do some additional work in the gym, all well and good.