It's just too much effort at the moment. Unfortunately walking for longer periods of time hurts my back in a way that running didn't, and for some reason I've felt no urge to go swimming recently. (Probably just the effort of getting and out of a swimsuit. I may look into getting a maternity tankini, one that is actually meant for swimming, as opposed to the one I have from Target from my last pregnancy, which can't have been designed for actual physical activity. Which is why I've been trying to get myself into regular swimsuits, albeit in larger and larger sizes.)
Between work and family this month (and every year at this time, I am so glad August is almost done with) I haven't even been trying to fit in much exercise, but even regular activity is becoming more effortful. So, time to let go of the dream of running throughout my pregnancy, and instead find ways to make walking and swimming more palatable, if not as satisfying. In six months I'll be able to start up again, and hard as it is to believe, those six months will actually move by pretty quickly.