Couple of thoughts on this subject. 1. Remember that when you're in a kayak, you can get closer to your target, so long casting isn't the primary focus. 2. If you need to increase distance, one way to do it easy while sitting in a kayak is to perform a water haul on the forward stroke. Simply allow your fly to "tick" the water on the forward cast, then immediately lift it, load the rod and make your backcast. Let the fly go on the the next forward cast.
I wrote an article on the subject of fly casting from a kayak for my blog: