I live in New Mexico and used to do the SF Century regularly. I don't think you're too slow, but that is going to be one hard day in the saddle for someone with the fitness that your average speed might indicate. For one thing, it's between 4000 and 5000 feet of climbing. The wind generally kicks up at the end of the day and it's going to be hot (already in the mid-80s here today.)

If I were in your shoes, I might consider the 50. Long way to drive for less than 100, but at least you'll enjoy yourself. The shorter route includes one of the big 3 climbs and adds another long uphill crawl that the 100 milers will be heading down.

It's a personal choice of course - I've seen people doing it on cruisers and Stingrays. But, I think you might be in for a bit of a struggle.