My partner and I just bought a couple of Advanced Elements kayaks and used them this season. We have the same arrangement, with our boarding ladder off the side. I'm not very flexible, (and am overweight
), but I was able to get in and out without tipping. We have covered kayaks, which makes it kinda tricky. If they were open, it would be very easy, but certainly doable either way.
I like the Advanced Elements Advanced Frame boats (we have the convertible and the expedition). They are not as nice to paddle as real kayaks (we used to do a lot of sea kayaking), but I'm quite impressed with the way they handle. They certainly do not cut as fine a line on the water, have more windage, and tend to flex in big seas, but heck ... they're inflatable!
Your back will get stronger if you paddle more, but good lower lumbar support is important. For Advanced Elements boats, I'd recommend getting the Lumbar Seat (which comes with the Expedition, but not the Convertible). I'd also recommend getting the optional backbone.