Speed work around an oval is a lot more mental than physical. I struggled through an epic 18x400m intervals with 200m floats. Translated, its not too disimilar to a 50 min fartlek with 1 min on, 30 secs off. Very tough going but frankly my legs have not felt this fresh in quite some time. Pace was not too strenuous - 400m intervals started at my 10k pace and worked towards 5km pace or a touch quicker. I was happy to call it a night before any fatigue set in. 

Now I've got JP Morgan Corporate Challenge (5.6km) next Wednesday. Surely the above session is beneficial, or perhaps too late? Guess we will find out in a few days...