keeping Heart Rate consistent

I have Edge 500, and the other day I set a workout and one stage I was to keep my heart level between 130-140 bpm. But after a exhausting exercise before that I just simply couldn't keep to that heart rate for long amount of time. So whats the best way to keep the heart rate within a zone for a certain amount of time?
ITS BY DOING WHAT EVER, THAT YOU BECOME WHOEVER!
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Slow down, might sound simple in theory, but in practice it takes quite a bit of concetration to do so.
Try and ride as flat a route as possible, as hills will push your HR up.
If you had got it right for the first time out you should have called in for a lottery ticket on the way home.
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If you want to be a strong rider you have to do strong things.
However if you train like a cart horse you'll race like one.
Despite the much lower HR today my average speed was down by less than 1mph - although the routes were different the average amount of ascent wasn't too different.
For me doing a ride mostly in zone 2 is going to have to wait until I'm a bit lighter and fitter I think. I found it also helps if there aren't many other cyclists about. The only time I saw other bikes going my way was on the last climb today - and that's also the only time my HR touched 160 as I caught and passed them (couldn't help myself as they were on mountain bikes!)...
cheers again