Agree with Sharla. You need a good, solid aerobic base before you can 'upgrade' your system through anarobic work. The aerobic system is the foundation that is always in use, the anarobic system is the turbo kick.

Now, if you have a good base... then you should focus on sprints/intervals etc. But that good base is REQUIRED. Unless you have a resting heart rate below 60, work on you aerobic base first.

just my opinion.