Grand Blanc Data

Above is the HR and Speed graph for the group part of the ride.
The total ride was 50.1 miles in 2:35:22

The final 15.1 miles of the group portion:
Duration: 36:35.0
Avg HR: 179
Max HR: 189
Avg Speed: 24.7mph
Max Speed: 39.3
Avg Cadence: 96
Max Cadence: 114

The variation in effort (indicated by the rising and falling HR) during these last 15 miles probably caused the most trouble.