my very reliable sources tell me that the dog was running slower than its aerobic/lactate threshold. clearly, then, it was using the half-marathon as a long training run for the full.
Nope, the dog ran as any dog would. Several sidetrips. Potty stops. Squirrel chases. Cat chases. Snooze breaks. The dog covered several more km than 21.1.
2:15 is over 10min/mi pace. When dogs run off leash, they will settle in whatever pace is comfortable for them. On a cold day, 13mi at that pace should be no problem at all for a lab.
Probably wanted to get a speed session in after that boring long slow run?