"12k" UQ loop

Today we ran as a group with Chris, our coach. The aim was to go 12k. We went across the bridge at UQ and looped to the other side. A hilly track and I struggled to keep up with the group - but I did! I couldn't belive when I looked at the time. 1.20.44. How could I have run 1.20.44 on a 12k and felt sooooo exhausted for the last couple of kms? We even had frequent rest stops to regroup and drink some water (I stopped the watch at these stops - the total run time was about 1h40). At the end, the truth came out. 12k? 12k my foot! We'd done 14k, thank you. Now I feel a lot better about feeling so exhausted, as we'd kept a pace of about 5.45min/km for the duration. To put it in perspective, 5.41min/km is on pace for a sub-2hr half. What else can I say to that? Wow, what a fantastic run!