Hi Steve - so in a case like that if you want to use Blend you have to split up the long edge into pieces so that you'll have a 1-to-1 match between edges on both sides.

You can use the Edit > Trim command to cut an edge up into smaller edge pieces - select the edge, run Trim, then click the "Add trim points" button and then you'll be able to click the points on the edge where you want it to be split at.

I will be working on upgrading Blend in v3 so that you won't need to do that anymore, but for now you've got to arrange your edges so that there is a matching pair of edges.

- Michael