This was Cassie responding last night to the Best New Thing in the World segment in which we met Jack Green, the British sprinter who wrote training reminders on his own arm to assure he remembered good running form.
I did a quick google and found a lot of silly running-themed temporary tattoos, but what I think Cassie is really talking about is something like this:
From a site called PaceTat.com, it goes on your arm (and comes off) with like any other novelty tattoo. The site says they offer different calibrations for different runners. The ratings look like this: 4:00 / 2:00 - 9:09 which means a 4-hour marathon, 2 hours to the half and a 9:09 minute first mile.
"Split goals" or "running splits" is what runners call it when they break a race into segments for the purposes of maintaining correct running speed throughout the race. So apparently a 26-mile race to a runner is more like a series of races strung together.
I also learned that there is such a thing as "negative splits," which basically means running the second half of the race faster than the first half. The idea is not so much to try to beat your first half pace as it is about not expending too much energy too early and then conking out too soon.