Even more variation?

I've been running on the treadmill to lose weight for the past 4 months or so. At first I could only do 5-6 mph, then 6-7, and now I usually do 7-9. However, I hit a plateau back in March. I've added weight training 2-3x a week and I've been getting "firmer", but I'm still at the same weight. (Btw I'm 5'4" and 136.)

As for diet, I've been doing  the zig-zag thing since my plateau and recently increased protein consumption, thinking that I might need to build muscle to lose more fat..I'm not sure if that's even going to help, but I've just been trying everything, really..

I'm wondering if I should try a different form of cardio altogether, and if so, what are your suggestions? I'd prefer something challenging (the ellipticals/bike never feel like they provide as good a workout as running).

Hey, I have some ideas.  My treadmill has an incline feature as well as tension variation.  I try changing these up from time to time.  Our bodies do 'learn' how to utilize energy/calories to perform certain tasks most efficiently.  This is why we need variety in our programs.  It also gets kinda boring if we don't.  It's for this reason I'd recommend maybe trying some new cardio routine for a few weeks, then changing it out again, to keep offering up some challenges for your body.  Also, for this reason introducing some strength training with weights would be excellent.  When you increase your muscle mass percentage your body will expend more calories doing the same activities because of the greater demand for blood/oxygen in larger muscles.  I do basically the same thing for cardio but I don't do very much of it, treadmill.  I rely on fast weight training for a lot of my fat-burning, cycling through my exercises quickly.  My heart rate stays at about 65-75% for the whole hour of training.  Weight training also offers other health benefits like healthier bones, heightened metabolism, and strength.  When combined with proper stretching such as yoga it can increase flexibility and supply more suppleness to connective tissues and joints, thereby lessening the tendency for injury. Plus it's fun.  I just love looking at my muscles when I lift!

If you want an example of what I do check out my journal entry "My Workout". 

Hey thanks a lot for the advice! I will definitely try running at an incline from now on. I guess I never did before because I didn't think I could do it, it seems really hard. By the way I actually broke my plateau this morning!!! YAY