> Considering the first mile would need to have been faster than 23mph

That said, note that it's not just the up-leg's average speed that's been provided (15 mph), but also the distance as you say (1 mile). If you explore the error term for both of these, you'll see that it's not necessary to go at the ludicrously high speed of 23 mph after all. """15 mph""" (15.444... mph) will do just fine.