Every training session should begin with a proper warm up. You've heard that before haven't you? So, why do so few people listen? Here are a couple of things to think about:

Do warm-ups prevent injury? - well believe it or not there is great debate in the scientific literature on this one - particularily the importance of stretching! Nobody debates the importance of flexibility in performance. More and more though are beginning to question warm up stretching's ability to prevent injury. For example:

No prevention of DOMS (delayed onset muscle soreness)

Research still needed, but improper or excessive stretching can lead to injury

Warm-ups make you work more efficiently - i.e. they make you faster!

Have you ever done a long steady state piece without a warmup? What happens? Every time, you find that your splits drop noticeably after a fairly predictable period of time.

In the old days they called it getting your "second wind." Now it's called being warmed up. The temperature of your muscles goes up, your aerobic system is kick started into gear. Metabolic ensymes start working more efficently. More importantly to you - your splits magically get better with seemingly less effort.

Don't believe me? Believe the research:

A 2-3% improvement in cycling performance
25% Lower lactate production in runners

You wouldn't race without warming up would you?

So don't train without one either! You're putting in all that time and effort - you owe it to yourself to warm-up.

Most coaches advocate a moderately long (15 minutes or so) period of easy steady state exercise. That might mean your 6k pace plus 15 seconds or so.

Some would suggest a few short bursts (10-20 strokes) are also important. They shouldn't come within three minutes of your race or workout though to permit the ATP-CP system to fully recover.