high intensity interval training. Basically 15-20 minutes of interweaving all out sprint efforts with average pace stuff. Something like
50 seconds average pace speed
10 second full effort sprinting
repeat 15-20 times
Word to this!!! And you can mix weights into this as well. Crossfit is a great example! http://www.crossfit.com/ check it out and I promise you will be surprised!
Best of luck!