I don't have an erosion node in my graph, no. Actually I found using the erosion node (as you suggested, just allowing it to process each patch for the entire terrain) fixed the issue.
What I believe the problem is is the exporter is having troubles loading in the full resolution of the input heightfeild files I have in the graph, if that heightfield wasn't already loaded in at full resolution (say by zooming the camera in close in the viewport).
I tested this with a single heightfeild in my graph, with the camera zoomed far out so only a low-res version is displayed. Exporter hung up and wouldn't export (at least, waiting 5 minutes). But when i zoomed the viewport camera in, and then tried exporting, the file was exported successfully in seconds.
So I tried then plugging an erosion node onto the end of my graph and allowing the node to process every patch of the terrain, and then exporting the terrain. Exported fine in just a few seconds.