To find out how many more miles Trey must run to complete the race, we need to know the total distance of the race.
From your message, it appears that "he has completed 14 of the race so far" may indicate that the total distance is 14 miles. If this is correct, and Trey has already run 1.2 miles, we can calculate the remaining distance as follows:
Total distance of the race = 14 miles
Distance already run = 1.2 miles
Remaining distance = Total distance - Distance already run
Remaining distance = 14 miles - 1.2 miles
Remaining distance = 12.8 miles
Therefore, Trey must run 12.8 miles more to complete the race.