My six pack is a one liter bottle

Or worse. I absolutely hate doing situps. Always have. I think my stomach muscles are either non-existent or so incredibly flabby that they provide no benefit whatsoever. I noticed something weird on my last 5K - at about the 2.5 mark, my kidneys started to hurt. Was I going too fast? Hardly. I figured it must have been a combination of fatigue, running on pavement (as opposed to the treadmill) and posture. Anyways, it went away once I crossed the finish line. But it got me thinking about focusing on other areas besides just running/splits/mile times. So I came across this article in Runner's World all about ab workouts, and how important those muscles are to getting the most out of your workouts. I'd like to start incorporating some stomach/back exercises into my normal weekly workout. Anyone have suggestions/thoughts/tips on that?