Turn around and drive back @ 60 mph. Your average speed isn't 45.
A problem that clearly illustrates this is a car traveling 1 mile @ 60mph. How fast does it need to go on the 2nd mile to average 120mph? Hint: it isn't 240mph.
Answer: impossible / infinite mph. 2 miles @ 120mph = 1 minute. You've already taken the full minute traversing your 1st mile.