First, we need to convert the 1/2 in 3 1/2 to a fraction with a shared denominator with the other fraction.
1/2 = 2/4 = 4/8
So, on a 6 1/8 mile trail, they have jogged 3 4/8 miles.
Subtract 3 4/8 from 6 1/8
6 1/8 - 3 4/8 = 2 5/8
They have 2 5/8 miles left.