So it turns out I actually am injured. Ran my second 20 miler in as many weeks yesterday, and ended up in a not good way. The shin splints (a generic description of symptoms, not problems) is caused by a soleus strain, like I suspected. And it's strained enough right now there's some visible bruising around the affected area. Couple that with the general soreness of my left achilles area and it's time to take a break from running.
So swimming it is. The good news is that I have plenty of time to rest before the marathon. The better news is that I am in great shape and if I can keep my cardio up with swimming (and cycling, something I'm planning on starting as soon as I can sell my motorcycle) until June I should be in great shape. I've got two 20 milers under my belt already. Even if I don't get any more I should be able to turn in a pretty good time.