Andy,
As your race is only 3 weeks away, you are better off not doing anything too short and fast.
One suggestion for a treadmill session:
Warm up and then set the treadmill at 2 hr half marathon pace (10.6 kph)
At the end of the first minute, raise the incline 1 degree whilst keeping the pace the same, at the end of the second minute, raise it another degree, at the end of the third another. At the end of the fourth - rest and recovery, lower the treadmill to flat.
Take 2 minutes easy jog or walk recovery and repeat. Over a couple of weeks you could build up to four repeats. This will give you the same benefit as doing a speed session but will be focused on the pace you intend to race at. It will also keep you within your 10k heat exhaustion limit.
Good luck!
Posted: 26/05/2009 at 18:27