I've gotten shin splints on long hikes - once 2 weeks into a hike, and another time 2 months. I wasn't able to determine how I got them though, which has really annoyed me! I get that I could probably prevent them with proper stretching or warming up or something, but I'd be just as happy to understand why I would get them well into a trip.
On the first hike, it was after a really hot day (40C+) but nothing was special about the trail, just pretty easy walking. My pack was fairly heavy for me, due to lots of water, maybe 35lbs. I hiked 30km that day.
On the second hike, I got them the day after a very, very rainy day. On the rainy day I had a quite light pack at ~18lbs, hiked maybe 40km.
I'm not sure what other details might be relevant. I would say in both cases I was in reasonably good shape, certainly well past the breaking in period of a long hike.