After reading a few of the posts, I keep seeing a lot of emphasis on getting measured for a pack. I am new to backpacking and I was thinking about taking my older brothers pack (he has a really bulky Kelty pack from the Army). Would this be a bad idea for a first timer?

Also, my first trip was going to be about an 18 mile loop with little to no elevation. I was going to make this trip in about 3 days. I am in fairly good shape and was going to keep my pack around the 30 pound area; does this sound feasible?

Thanks,

Mike