If you are doing the same 6 miler in Eagle Creek, there are quite a few ways to mix it up in the park to get different loops in. If you just run the outermost trail loop (including the land bridge out into the water) from the 56th street gate - that loop is around 7-8 miles or so...and then there are ways to loop back on inner loops in the park to make runs longer or get different 6 milers in...
Monon has been mentioned and that might not be bad for a change of pace I guess...
Canal Towpath between Broad Ripple and 30th Street could also be a good option...
I don't know much about the trails out at Ft Ben - do they have "soft" trails? I only know the asphalt one that is east of the pond...it isn't really that long...