It took her 8 minutes to run one mile.
Explanation: Since she ran at the same rate the whole race, taking 3h 28min to run 26 miles, we simply divide the total time she took by the miles she ran, to find the time she took for one mile.
Converting the time into minutes will make it easier, so: 3h 28min = (3 * 60) + 28 = 208 minutes.
So one mile will take 208 / 26 = 8 minutes
Let me know if you have any questions.