Look up fartleks. Running in a group was the best way for me. Indian Run/Army Runs were my favorite. Also, plyometrics will kill you in the best possible. I have P90X and it is a great work out, but the guy is annoying as fuck and you just want to kill him the whole time. I think if you go to Crossfit.com, they list a different work out to do every day to keep things new and fresh.