To find out how many more miles Trey must run to complete the race, we start by determining the total distance of the race.
Trey has completed \( \frac{1}{4} \) of the race and has run 1.2 miles so far. To find the total distance \( D \) of the race, we can set up the equation:
\[ \frac{1}{4} D = 1.2 \]
To solve for \( D \), we can multiply both sides of the equation by 4:
\[ D = 1.2 \times 4 = 4.8 \text{ miles} \]
Now that we know the total distance of the race is 4.8 miles, we can find out how much distance remains. Since Trey has completed \( \frac{1}{4} \) of the race, that means he still has \( \frac{3}{4} \) of the race to run.
To find the remaining distance, we can calculate:
\[ \text{Remaining distance} = \frac{3}{4} D = \frac{3}{4} \times 4.8 \]
Calculating that:
\[ \frac{3}{4} \times 4.8 = 3.6 \text{ miles} \]
Thus, the number of additional miles Trey must run to complete the race is:
\[ \boxed{3.6} \text{ miles} \]