He goes 3 feet from the basket, turns around, walks another 3 feet
back to the basket.  That's 6 feet he's walked until he returns to
the basket the 1st time.  

Then he goes 6 feet from the basket, turns around, walks another 6 feet
back to the basket.  That's 12 feet he's walked until he returns to
the basket the 2nd time.

The series is

6 + 12 + 18 + ...

It has 100 terms. n = 100, a1 = 6, d = 6

Sn = [2a1 + (n-1)d]

S100 = [2(6) + (100-1)*6]

S100 = 50[12+99*6]

S100 = 50[12+594]

S100 = 50[606]
 
S100 = 30300 feet or 5 miles 3900 feet, about 5.7 miles.

Edwin