If you travel at 60 MPH for 10 miles, and 30MPH for 10 miles. What is the elapsed time for the 20 mile travel? 30 minutes. The average speed is not 45 MPH though. More like 40.
Now comes the part that is harder to gauge, If boosting along at a set ground speed in rolling hills was actually a longer distance than the airspeed -the total velocity you loose by not being able to dash, over a long enough distance it might actually be faster to fly.
If you travel downhill at a ground speed of say... 100 game units per second then along the flats at 100 per second, but uphill can be slower (not sure but i suspect in game speed is units per X axis and an additional y axis the same speed can be added if you had wings) Unless it was possible to gauge actual ground speed against airspeed (true X/Z axis speed?) then it might never be known if it's faster to fly uphill and run downhill.