How many miles should I ride each day to prepare for a 66 mile ride in 6 days time?
Honestly?  6 days is a bit late to do anything significant.  You should probably do ~5-15 mile rides for the next 4 days, and have at most a short 2 mile ride on day 5, and then you've got your event.

Try and do your rides on similar terrain/roads if possible.   Ideally you'd do pieces of the real road - they always feel "shorter" on subsequent trips.

Don't get carried away with replacing major mechanical components this close to the event.

Your best bet is to do basic bike maintenance, like lubing the chain and tweaking the gears and brakes, and inflating your tyres.  Check out your spares and on-bike tools, make sure its all good and that the spare tube has not acquired a hole.   

Also - if your event is self-supported, you'll have to carry 66 miles worth of water and food.  You'll need to pack that onto your bike in a way that doesn't interfere with riding, be it camelbak or frame bottles or panniers or whatever.

If the trip is supported (ie, feed stops every hour or 20 miles) then its lower importance but still don't run low.  Sometimes the fueling stations aren't there or have run dry, (or blown away!)

Finally, plan on how you're getting home.  Noone wants to ride another 66 miles back home after such a trip!