The standard running loop around Lake Union is a touch over 6 miles. With the addition of a side loop around Portage Bay, you can bring it up to 8 and a half, taking in a bit of UW's campus and crossing over the cut into Montlake. Sticking to the water's edge keeps the terrain nice and flat, but if you want some climbing, head up into Capitol Hill via Interlaken park.
Here, I've factored in a stop at PCC for a cold drink.