Olympic medallist Chris Boardman on the secret of Pooley's success
And it looked as if Pooley, who played a key role in helping Cooke claim gold at the weekend, had done enough to win gold.
Her time of 20:46 seconds at the 10.8km time check was unbeaten, with eventual winner Armstrong the only rider to come close by recording 20:50.
But Armstrong managed to make up the remaining time during the descent and came home just over 24 seconds ahead of Pooley to claim gold.
"In the road race, I was doing my job," said Pooley.
"Today, I just concentrated on riding as fast as I could, being absolutely smooth and enjoying myself without anyone to push me off."
She added: "It helps being positive-nervous, because you've been waiting four years for this.
"I was enjoying going fast. It was pretty fun, I just kept saying, 'faster faster.' I raced as well as I could. I cannot affect what others do.
"There's no secret, you just have to make it hurt. Imagine a friend sitting on your wheel, shouting at you."
Video - Pooley overwhelmed with silver medal
Cooke paid tribute to Pooley after the race, telling BBC Sport: "Emma worked so hard and is a very worthy medallist, it just shows what a strong cycling team we have.
"She was a very critical part of my gold medal on Sunday and it's absolutely brilliant Emma's got her own medal today.
"It was a very hard course that required maximum effort throughout.
"My legs felt fine going in, but when I started I knew it wasn't going to be a great ride. I just tried to dig deep - the race is never over until it's over."
Armstrong was overcome with emotion after winning gold, a victory that made up for the pain of missing out on selection to the 2004 Athens Olympics.
"It's the most amazing day of my life," she said.
"I've been working for this for the last eight years, especially the last four, and to time everything right on one day is an accomplishment of its own."
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