I just mapquested it... they went I-77 N toward Cincinnati, then some state roads across Ohio. Their route was 585 miles just generically picking "charlotte" and "ft. wayne".

If you want to head back across 40 to pick up 75 there are 2 minor options that would save a little time.

A. start on I-85 S then pick up 321 N out of Gastonia (in NC about 20 miles west of Charlotte). It will take you to I-40W.

or

B. As you are going through SC on I-85, look for ( I believe) it is Rt 11. It cuts 2 sides of a triangle off I85S - I26W.

Chuck