@bud_t in that case, I think your best best is running a few blocks down to Alaskan Way and then turning right and running along the waterfront up to Elliott Bay trail and Myrtle Edwards park, turning around when you've done 2.5k. There's no particularly nice route from 6th to Alaskan I'm afraid, but the waterfront is mostly pedestrianised, you'll go past the Sculpture Garden and the trail in the park is pleasant!