Gah! You are mad! I ran (just) the Coventry half a few years ago, the week before Snowdonia, and did I ever regret it. In fact, I stopped twice involuntarily on the Snowdonia, and felt like crying (I didn't though, let's just get that clear, right?). Snowdonia is a tough old race, even without a tough race the week before!
The Greensand will be quite some warm-up!