Pilates.
Even early on in Pilates, you learn a lot about lower abs. It's a refinement of "crunches" I believe, takes a bit of learning, does not involve the six-pack, but once learned, does not need 35 miles per week, nor particularly horrible dieting. I say this because it's made a noticeable difference for me, and I'll never hit 35 miles per week, and I've never successfully followed a slimming diet in my life.
And its good for running as well. (Strong core makes moving the legs easier.)