As Droid said, you can put your mind at rest. Your car is fine. Just fill the car until the pump shuts off. Overfilling is when you go past that point.
On your 120 MPG adventure, I would say that as capabile as our hybrids are, they cannot overcome the laws of physics. A car cannot move uphill without energy being applied. In the 45K miles I've driven my Civic, I've learned that it takes a reasonable grade downhill to coast at 120 on the display and maintain speed. You still need to overcome wind resistance and friction from the tires. I believe what you experienced is from the fact you just filled the tank.
There are many times where you'll get brief runs or even a tank where the mileage is very high or low. I've posted before on the dozens of variables at play that will affect your mileage (wind, rain, hot, cold.....). Don't get too worried about what happens over 1 mile. I suppose if you had a 40
MPH tail wind you might be able to go uphill on 120. :-) In general, what your car does over a month or a season is a better measure.