I'd go ride back-to-back 100k rides every chance I had in the meantime and go for it.

"Slow" is relative. I just finished a long ride, and we averaged 15.02 mph. So you're a bit slower than that, but not a bunch. At 65 miles, you've got 5 hours ride time. In Minneapolis at that time of year, you've got something like 14 hours of daylight. If you're trying to stay with a group, you may have problems. If you're willing to ride alone, or have someone willing to ride with you, no problem. If you don't have anyone willing to ride with you, ask around, it may be easier than you think to find someone. Personally, I'd rather ride 10 mph with someone friendly and talkative than to ride 20 mph by myself or in a paceline where all I see is another person's tire, and there's some other people out there that think the same way.

If you have overall exhaustion, I'd consider seeing a doctor and seeing if anything is fixable. If your legs are sore, that's pretty normal, though. What I find is that if my legs are sore, they'll stay sore for 2-3 days if I just sit around the house, but if I get out and ride, then they'll be okay.

If you live someplace hilly and the ride is in a flat area, that'd actually help you. If your route will be all day in the same direction, you can potentially ride into a headwind all day.