first off, what ride are you planning? Secondly, you CAN do a metric right now... might hurt, but you can finish it.
My first metric was in Chester, SC... not as big of hills as you have in Spartanburg, but my rides prior to the metric were 5 miles, 5 miles, 10 miles, then the metric. I did it 10 days after my first ever ride on a road bike. I wasn't the speediest at it, but I finished. AND, even better, didn't need help to get out of bed the next morning which was as big of an accomplishment in my mind as finishing the day before.
so, go for it. You will finish. Your butt will hurt, but you'll have fun and will get a much greater sense of accomplishment from it than just riding the 28.