Laura L's training comment yesterday got me thinking about this and I'd like the guru's out there to confirm or correct...
I thought there were two types of interval session
1) Aim develop a finishing kick.
ie after a Long distance race you wnat to be able to change gears and leave a trail of rubber on the road as you catch folk over the last 400m.
For this you need to be able have a maximal capability to combine aerobic and anaerobic power even when the body has lots of lactic acid around but only for a short duration.
to acclimatise to this demand you do fairly short intervals (2-600m) with short recoveries and lung bursting pace.
2) Aim to develop the ability to run faster for prolonged distances ie cut your time off 5k/10k/half M.
You need to aid overall CV development and lactic acid tolerance over prolonged time and increasing level, but not extreme power as per 1)
Training is therefore geared to intervals at a bit faster than race pace (eg 110%)and over extended distances 800m+ with reasonable recoveries. the aim being to cycle through the lactose levels.