I went to the Rec at lunch, and it was packed! It cleared out by around 12, so next time I'll go then. Since the treadmills were full, I started with 3.5 miles on the bike (= 1 challenge mile). Then I got on the treadmill and did Couch to 5K Week1 plus for 2.12 miles.
Here's the link to my Nike+ graph. I don't know why it doesn't show the intervals more clearly. I was switching between 3.8 (walk) and 4.7 (run).
Edited to say I fixed it. Thanks, Laura!