Someone please check my math.

4.1 mph up a 12% grade that is 4.1 miles long would take 1 hour.
Descending that same 4.1 mile long 12% grade at 52.4 mph would take about 4 minutes 40 seconds.

Total time to travel 8.2 miles is 64:40 and an average speed of about 7.6 mph.