Here is what I tell all my customers get your work done a month or so before your big ride. Waiting till the last second is never a good idea because if something goes wrong which can happen even under a skilled mechanic who has done the process before for various factors and reasons, you don't have time to get it fixed. You will also not have time to ride the bike and make sure all is good. You want to allow time to ride the bike and bring it back in, if it needs to be touched up and some places cannot always do under 24h turn around especially on specialty items that need parts they may not stock and also because other people have come in beforehand.

I would just ride the other wheel and use a cheaper wired unit as [MENTION=38165]HillRider[/MENTION] suggested. Just enjoy the ride and don't worry too much about your metrics. We frequently get caught up with our devices and being connected that sometimes it is nice to just ride to ride. You will still get a work out and still get some training in and plenty of pros trained without any power meters and gotten great results (Eddy Merckx had 525 really great results over his career on a steel bike with downtube friction shifters and non-aero brake levers)

Have fun at your ride on Sunday. I am hoping my ride won't be rainy because a lot of folks are going to it from all over.