How long is a warm up?

I swam my usual mile before work - wondering whether I should be changing my breathing technique so as to swallow less water. It doesn't matter much in the pool, but when it's a lake... probably not that clever to swallow half of it! So I practiced different breathing and decided that actually I'll just stick to what I know this time, but if I plan to do another triathlon next year then I will Make Improvements. I had also worked out what time the winners took to do the distance and set the pool speed that high. Ha ha ha ha ha! The words 'fat' and 'chance' spring to mind - so I'll stick to the speed I know I can do too...!

After work (well, half a days work today, then 3 hours spent at the Apple store trying to get a bit to work on the computer... which it doesn't, very frustratingly) home again to pedal my 10k. Should I be doing warm up exercises? It seems like really hard work for the first 7k, then it all gets easier. So if 7k takes me about 20 minutes, do I have to do 20 minutes exercise before I start pedalling? I'll be worn out before I start! Maybe it's something I'll look in to... And I need to listen to something more interesting too - the radio tonight was seriously dull and the 35 minutes felt longer than it needed to - I even longed for Patrick Kielty!