There are four levels:
Junior Varsity, Varsity, All-American, Heisman
The game defaults at Varsity. Junior Varsity is a turkey shoot and meant only for people who have never played before. Varsity is a good step up. It usually provides maybe a smidge of challenge, but any decent player should be winning evenly matched games by 4 TD's. All-American is very challening. The AI for the other team is smart, and you have little room to make mistakes, i.e. throwing into coverage will get you picked off 70% of the time. And Heisman is just ridiculous.
Freak -- You just gotta practice with the option to get it down. Usually it's best to run when you're pretty sure you won't get a hardcore rush thrown at you. Make sure you've got room to run. Don't pitch it off the bat, your runningback should follow you at good distance. The linebackers or corners will usually sniff this out, and once you can get them to commit to the tackle, pitch it at the last second and your running back should be open most of the time.