If I'm honest its now 6 months to the day since my c secion and I don't feel totally comfortable doing a sit up now.
They say 12 weeks till you do any proper exercise, but if you're already a gym nut then you'll probably recover a lot faster than I did - I stopped going to the gym about a year before I feel pregnant, and I overdid it early on afterwards, so my recovery time is dragging along at a snails pace. Its very much how you feel, but as soon as you feel your scar pulling its time to sit back down as you've gone too far. I would suggest leaving any exercise apart from gentle stretches till 6 weeks. If you can't drive a car you can't go for a run!!
Here's a good exercise for when you get to 6 weeks. Lie on your back with your knees bent up as close to your tummy as is comfortable. Lie baby on your shins on his tummy facing you, and try to straight your legs keeping hold of baby balancing on your shins. He'll love it.
Try finding a baby yoga class - it'll sounds daft for someone who's used to doing weights and the cross trainer, but its good fun for you and baby and you'll meet other mums in your position. Really recommend that one!!!
Swimming you can do pretty much as soon as you feel ready, but start off doing just a few lengths the first few times as it won't be till you're out of the water when you realise you've done too much. Realising you've done too much is not comfortable and the backwards step is really frustrating. Have you considered Aqua fit classes??
Whatever non swimming pool exercise you do try, stick a cushion over your tummy - its psychological but it really does help make you feel safer.