Pressure is not nice

I'm in the middle of reading  A Life Without Limits - A World Champion's Journey by Chrissie Wellington.  I'll write a full review for our friends at Trijuice.  Sitting out in the Arizona desert after a brief, but very hot run, one passage jumped out:

"Pressure is not nice.  It is a necessary evil if you want to achieve, but you're not supposed to enjoy it.  It brings with it great stress, but you deal with it, and the redemption comes when you achieve things because of it.  The trick is to understand which pressures are necessary and which ones are the dangerous decoys..."

When should you push that hard tempo run?  When should really push for that charitble cause that is close to you?  When should you demand excellence of yourself at work?  Maybe these questions are always one and the same.