I started running (very) seriously only recently. I can tell you for a fact that weight training will not significantly improve aerobic fitness. However, it wi

I started running (very) seriously only recently. I can tell you for a fact that weight training will not significantly improve aerobic fitness. However, it will improve it up somewhat. Just an example, you will never run 5km under 20 minutes just by weight training. But you can run 5km on the first try (say.. ~30 minutes) if you are into weight training (so your cardiovascular fitness is okay-ish, it's just not great). Almost the same exact thing happened to me, as someone in supreme physical shape. Long story short: you get great results in the things that you directly train; some things overlap, but the overlaps are never significant.