I just saw this, sorry I realize it is an older post.  Well, this is what I do.  One day a week, I do intervals on a stationary bike which kills!  It is so much harder than I thought and actually a lot like running.  I do this instead of a run once a week.  Then, once a week (if the weather is lousy) I do intervals on a treadmill instead of a run outside.  Two days per week, after the cardio or a run, I do strength training using dumbells, exercise balls, etc.  I have noticed that running has gotten a lot easier since I started the strength training.  I do relatively low weights, but high reps (25-30 reps).  Believe me, it is HARD!  Well good luck!