3 day, about 25ish mile backpacking in Sept??

 My partner and I are heavy in to training for a Grand Canyon rim to rim hike in October.  While we're experienced hikers, we're new to backpacking!  (Only 1 trip so far!)  We thought getting in 'canyon length' backpacking trip (25 miles give or take; 4500 total gain) over three days w/pack on would be good training .  . .     and great to get one more backpacking in before it gets too cold!  Ideally, not to far out of Portland (2-3 hr max).  And although Eagle Creek sounds great. . .  being away from crowds is a high priority.  (Not sure how it would be mid Sept?)

Ideas or Input sought and thanks in advance!

Sheryl