All depends on what your goals are. If you want to run fast(or, even just a significantly improved) 5K, 10K, half marathon, marathon times, then you need to put in the miles. Running some high intensity intervals a couple times a week isn't going to help you knock 5 minutes off your 10K time. Sure, if you are just looking to lose a couple pounds, then I suppose you could do it with weightlifting only.