With that 26-34 combo, your lowest gear is already close to walking speed -- 3.7mph at 60rpm.

I don't think you will really benefit much from going any lower. Plus, let's face it, the lower your gearing, the more likely you are to dive for the granniest of your granny gears once you hit a hill.

You may just be better off doing a lot of hill repeats in a higher gear in order to build up those muscles. As long as you're still spinning rather than mashing, your knees should be fine.