Hello there!
We first do (6+5=11). = 1 practice
11+11=22= 2 practice
22+11= 33= 3 practice
33+11=44= 4 practice
44+11=55= 5 practice
If they ran 57 miles, and we only have 55, we known that if we go anymore, that would be inaccurate, so based from what is understood, we would estimate this, and they only have 5 practice days.
5 practice days would be your answer!