1/3 Century or Metric Century?
You're asking should you try to ride 33 miles or 65 miles when your longest training ride is shorter than either?

I think you'd be much better off trying a 60 mile training ride before the event. Sure, you could probably do that on the day, but you could easily end up either failing or struggling through and feeling miserable.

If you think the "supported" part will make a difference perhaps try carrying a bit of cash and choosing a ride that lets you stop a couple of times at shops where you can buy snacks and drinks. Personally, I'd carry both on the ride. A extra couple of pounds isn't going to make any difference unless you're right on the line before complete exhaustion stops you. And you shouldn't be there.