Mike
A lot of my running is done as part of my commute to work, so I run with a backpack containing my laptop, work papers, a towel, etc....
This definitely makes these runs harder, and I notice the distance and pace much more than when I'm running without a pack on.
I'm not suggesting you run with a pack on solely for the training benefit, and I have no clue where your training is upto in terms of volume or quality, but it might be worth thinking about how you can mix up your running to get some different stimulii?