I think the single hardest thing is not the technical difficultly of the green and blue trails, its the speed. Without trying or even pedaling, you will be going way faster for a longer duration than you would go even on the steepest parts of Ringwood, Waway or JH. That said, if you want to go(and I think everyone who does xc should try it at least once) rent a bike and go for the guided rides that they have once it opens daily(check the website, they have some deals where you get 2 hrs, helmet, armour and bike for 60 bucks or something like that)
on a side note, I do think some trails are mislabeled there, Lower greenhorn is more difficult in my view than alpine, I think alpine is labeled a single black diamond because of the wall rides and jumps, but you can skip the wall rides and roll all the jumps. Lower greenhorn has a couple of very tight turns that are hard to take with any speed.
Go and try it, dont be surprised if you find yourself addicted and looking for a dual crown beast on pinkbike/ebay after your first time.