So I assume the horse you will be borrowing is a trained barrel horse? If so, that is a good way to start with a horse that already knows its job so that you can focus on learning as a rider. But with that said, of course it cannot be a horse that is too much for you to handle too fast. Best thing is to start slow, learn correct, get it into your muscle memory, and only pick up the speed when you can keep your riding form.
Along with working with a trainer, I would also recommend you read/watch up on books and DVDs. Charmayne James, Martha Josey, Dena Kirkpatrick, Sharon Camarillo, Ed and Martha Wright, etc are just a few good names to look for. You can also find a few free videos on YouTube by Charmayne, Dena, and Martha J. There also are some decent ones by Expert Village.
Running barrels is a lot more than racing around 3 barrels as fast you can. You have to consider your pocket (how you set your horse up for the turn), keeping yourself stabilized in the turn and your body position correct, being able to control your horse in the turn and make any corrections in a split second, and just understanding the dynamics and physics behind a fast run.
Barrel racing is a lot of fun, but do realize it's work ahead to do it right! Good luck!
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.