1) Pastures are tough because it'll tear the crap out of it. You also leave divets in around the barrels and it makes it harder on the horse around turns. On top of it all, it is slightly risky because you never know if there will be groundhog holes or anything. But as long as you beware of all of the able you should be fine; assuming your pasture is big enough for barrels.
2) Start by setting up the pattern and ground walking it to be sure there's nothing in the way that may hurt your pony. I'd suggest first walking each barrel. Then when she's getting around them with a nice turn (and ease) i'd trot each barrel. Do figure 8's around 2 barrels at the trot and practice getting close and riding her around each turn. MAKE SURE her hind end is doing work as well. After she's bending and moving around each invidual barrel try trotting the whole pattern as if you were running it for real. When she and you both are confident and balanced around the pattern, do a nice lopey canter and be sure she's making her turns and listening.
After all this, you can increase your pace. But keep in mind, speed is not everything. Running as fast as you can won't get you around the barrel if you're unbalanced and rushed. Also this training should not all come in one or two days. This is going to be hard on her and she should build up muscle and strength before she's ran through a true pattern :). Good luck with her!
Count My Strides - Shane, Bought 2-18-06, Mustang.
"I whispered to my horse, "i'm afraid of falling" and he whispered back, "i have wings".