Step 1: The runner has completed 12 km. This 12 km represents 80% of the total race distance.

Step 2: Let the total distance of the race be denoted as 'x' km. We know that 12 km is 80% of the total distance. This can be expressed as: 12 km = 80% of x

Step 3: 80% = $0.8$

Step 4: 12 km = $0.8 \cdot x$

To find 'x', divide both sides of the equation by 0.8: $x = \frac{12 \text{ km}}{0.8}$ $x = 15$ km

The total distance of the race is 15 km.