The ULTIMATE resource for rider fitness/workouts, is called The Rider's Fitness Program. It is a book availible at most tack stores, and on Amazon.com. I absolutely love it. It has tons of workouts especially for riders, and it would be very helpful for you.
Also, check out Practical horseman magazine monthly. They usually have at least one rider fitness article a month. Its like Seventeen Magazine for horseback riders. Great.