Yes, it'd be good for a first century; I had done a few before this one, but had several friends who used this as their 1st century before riding the STP. As ttaylor said, it's a mild grade for the 1st 50 or so - and you'll get to the Fish Hatchery rest stop, which is the turnaround point for the 50 mile route. Then it's a pretty steady grade up to the century turnaround near Silver Falls but, no, it's not a steep mountain pass at all. The return is grand as others have noted, many lovely downhill glides, then back out on hwy 97 and you'll more than likely meet the renowned wind - hopefully a tailwind, but I've experienced more head and crosswinds on that afternoon return.
There are a lot of pacelines early on, so you'll have a bit of a pre-STP experience in the early hours of the ride, so you can practice keeping your line, talking to each other and riding safely together. Also, I think I mentioned it before, but this is 'goat head' country, so don't ride all the way to the right, as a friend of mine did and had 2 flats from them in the first 25 miles of the ride (on Entiat River Rd). It may be a nice cool morning to start, but it'll probably warm up quickly. There are plenty of trees for cover on Entiat up to Silver Falls, but it's pretty exposed on hwy 97, in the heat of the afternoon, so slather on the sunscreen and load up on your liquids too. They have usually parked vehicle for a water stop on the early part of Entiat River rd, between hwy 97 and the fish hatchery stop, so I'd recommend looking for it, to mark it on your outbound ride, assuming that you won't need it then, but it'll be good to keep in mind - and use - later on.
Have a great time!
Mary

