I have no business answering, certainly not so early in the thread. But I'll answer anyway so as to generate a subscription.

I'd say give yourself a week or so of moderate rest. Don't totally slack off riding, just don't ride to near exhaustion. In other words, skip the weekly long ride for a week.

My limited experience with long distances (and I've certainly never done 120mi) says that if you can do 90mi you can surely do 120mi. The key is what and how often you eat and drink as you go. If you're comfortable on the bike and have developed any sustainable endurance, the extra 30 miles will be nothing special.