I would assume a good pace is the pace that can be sustained through out the distance without extraordinary difficulty or injury.

Since there is no accounting for differences among different routes and weather conditions, I guess a good pace would be reflected in the riders ability to control his or her power expenditure is the wisest and most efficient manner.

Therefore, for a Century ride, a good pace is a dynamic, ever changing use of the rider's power in conjunction with the group he rides, the terrain and environment he encounters, and most of all his finish time goals or requirements.

In some ways a Century ride is like riding a bull - you never know what the ride will be like until it's over.