Nearly 8K feet of climbing, at least from my experience, is a decent amount of climbing for 100 miles. However you should not focus on if that "is a lot or not" in the view of others, because it depends on everybody's individual capability as well as where they are located in. I have done nearly that amount in only 25 miles (Haleakala in Hawaii), and that was pretty brutal, but I have improved a lot since then. The Auburn Century around here has a 140 mile option that I've signed up for that does 17000 ft of climbing.