Here is the initial design idea I came up with for your runs--it's pretty straight forward: runners in costumes running in the moonlight. I kept the colors flat/solid, so you can go straight to the printer for silkscreening onto shirts. This particular version is a 3-color print (orange, white, purple) on black. The colors are quick and easy to change, as needed, so feel free to let me know if you have any specific palette nailed down yet or not.
Let me know what you think in terms of color and the content, and we can go from there. Thanks for looking!
I also added an optional distressed/textured layer, which can be turned on or off to give the shirts (or anywhere you put the design) a vintage feel...