I always have a bolt of inexpensive muslin on hand that I use to make patterns or experiment with things. This is one place where I would use that to try adding 8", subtracting etc. Once I had it right then I could use that as a pattern to cut out my nice fabric. I never use the muslin in the finished product as it isn't good enough but it is perfect for making patterns. I can write on it and it sticks to the fabric so few pins if any are needed.

The original pattern is by Karen Montgomery and uses two strips of striped fabric and when you cut out the triangles it is approximately the length of the strips minus the height of one triangle. The pattern that you refer to (if it is the one I found on Google) is a take off on her pattern and uses three strips of fabric. I have used the original fabric many times - finished size is always shorter than the original two strips. It is a great pattern and works up quickly. Good luck!

Is this the one? http://melindasfabricshop.com/TQC272/patterns.html

I have that exact same pattern and I found that part confusing I almost did the same thing you did that minus 8 inches is after you cut the triangles look at the green and white diagram in the instructions,that paragraph of instructions is misleading at least to me, makes you think you should cut your fabric at 8 inches less then the size you want but that's not the case it took me a while to figure that out DUH! Me it is a very easy pattern hope you post a picture

Shhhh you have my issue if broken ruler lol. I'm still trying to get it right. Mine end up napkin size!

Perhaps you could add a pointed piece to the runner, which should add enough inches to the runner.

When you put the short ends together, you get a V on each end. When you add the triangles (2 to each end) they stick out 4" more than the longest part of the original strips, thereby adding the 8" back. These 4 triangles are cut out with the perimeter stripe on the bottom (I call them "ups"). The triangles in between the cuts are reversed (I call them "downs").
If you can get this to work, you can make another one with the stripes reversed and use the "waste" triangles (the "downs") you cut when you got the 4 triangles for the first one.
I am making something with UPS and DOWNS, but put them together to hopefully explain.
You can see when the triangles are added to each end it gets longer. Hope this helps. Ann

Yes, that is the culprit

Finally succeeded. Ended up using a different technique that ended up looking the same. Now to get it sandwiched and quilted. The first attempt isn't a waste as I need a piece for the podium. Thanks for all the suggestions.

Ann, that is pretty fabric. Will be a really nice runner. Marcia in TX