We just go around the block - about 0.6 or 0.7 miles - and we don't run the entire way. We have what she calls "second goal points" to which we run, and then we walk for a little bit before running to the next point. She says she doesn't want to run any farther than that. Just around the block. But she wants to run around the block every day for the rest of her life.
I don't know how long this will actually last, but I'm doing my best to keep her interested and to avoid putting pressure on her. I do know that it's nice having a running partner, and our trips around the block make for a good warm-up before I go out for longer runs.
The culmination of these interests is that she likes to dress up as Die Flederkatze (a cat version of the hero, Die Fledermaus from The Tick) when we go running (as well as when she's just hanging out around the house).
She has also decided that we will all be cats for Halloween this year. But while I do have a set of ears of my own now, thankfully she has not insisted on having me wear them when we go running.