You really need to get more vigorous exercise than walking, especially for IBS-C. The diet, supplements, and medication can only work so much for C. You need to speed up your system. As an IBS-D I can barely get enough exercise to consider myself healthy because it makes me 'go' too much. Also don't be nervous about it causing cramping or pain, I find that while exercise makes me 'go' more (sometimes too losely as well), it has never actually caused an attack or pain to my stomach.
Some cheaper ideas than getting a gym membership: Jump rope - super good workout Jumping jacks Running - during your walk try running for a short spurt to start with Ab workout - crunches until you can't crunch anymore Stair stepping - step up a few stairs, and back down, kinda like stairmaster except its free Yoga - look at the poses on this website that are good for relieving gas