What is a "Fartlek" you ask?
A fartlek run means "speed play." You should warm up with easy jogging for 1 mile or about 10 minutes. Then you will run harder than normal pace for 3 minutes, then run slower than normal pace to recover for 2 minutes. Repeat this 4-6 times then cool down 1 mile. For example if you run a 10 minute mile for most "conversational pace" aerobic runs then for your hard 3 minutes try to run between an 8 to 9 minute mile. When you recover for 2 minutes you need to slow way down -- to an 12 minute mile jog -- or you will not be able to run hard again when it is time.
Here is what "aerobic" means:
Aerobic: A moderate paced effort where you push yourself, without getting exhausted or completely out of breath. Your breathing should be in control and you should be able to carry on a conversation with little difficulty. Your long runs are run at a “conversational pace.” You should be able to maintain this effort your entire workout after an easy warm up. Heart Rate Zone: 70-80% of maximum heart rate.
A Tempo Run?
Tempo Run: After warming up with 2 miles at an easy pace, increase to AT pace for a specified time period (usually 20-30 minutes). Control your pace so that you push yourself, but not exhaust yourself. Cool down for at least one mile afterwards.
AT Pace:
Anaerobic Threshold (AT pace): A harder effort where you are pushing yourself yet still running efficiently. This is not an all out effort; you should have another gear to go if you have to pick up the pace. Used primarily for longer interval workouts or “tempo” runs as defined above. Heart Rate Zone: 80-90% of maximum heart rate.