I took some advice to find a 'moderate' hill (maybe one in ten gradient), and run up it for 40 seconds at a pace where, in the last 10 seconds, you could not possibly hold a normal conversation... but you would definitely be able to say the odd phrase.
Then jog very slowly down and repeat the cycle 5-8 times. It seems to work for me.
That way, you're in control whether you're a seasoned runner (where you need to run quite hard to get out of breath) - or a relative newbie - who might just be doing a fairly gentle uphill jog.
It's important to warm up with a gentle run of a couple of miles first. You don't want your heart & lungs to go from cold, to suddenly launching into full power.
Good luck. I'm convinced this exercise has helped a lot for both 5km speed work, and longer endurance stuff.
Edited: 04/10/2012 at 10:36