GS is excellent. So is the Slow & Heavy.
You might consider getting the 4 Day Split, if you don't already have it, and doing all the lifting from that series, splitting it out over three days, same as a traditional 3 day split. I have only heard good things about the lifting program on 4DS, and you would also get 4 great cardio workouts for your money!
You may not want to hear it, but doing cardio other than treadmill workouts would bring enormous benefits and would ramp up your fitness, not to mention, bust you out of that treadmill rut. Try it!
Clare