I'v just been reading an interesting article about coping with high intensity exercise for long periods of time.
http://www.pponline.co.uk/encyc/0991.htm
Some sports psychology for you all

I was interested in this because for example on some days i can run for 18 minutes on a 1% incline/ 16km/h but on other days i can only manage 12 minutes.
This takes practice as I tend to concentrate on how awful the pain gets in those last few minutes rather than trying to concentrate on other things. This might be useful for those attempting a PRMC soon (like me).
Let me know how it works out for you all. Its all about positive thinking! Never think about "oh crap, my legs are lead weight today!!"