We did it! The 5K was this morning. My husband and I ran together and finished together. It was fun doing it as a group and as a fundraiser.
The course had a few hills and one rather large one. I wasn't sure I could keep running at that point. But I got up it. I pushed that stroller all the way up - jogging. Very slowly, but jogging nonetheless. At the bottom of the big hill before I went up, my HR was 155. At the top it was 178! And then on the way down, the stroller was almost dragging me. And my HR recovered to a respectable 145.
After, my husband said he really enjoyed himself. He said that now he understands why people run 5Ks: "Before, I used to wonder why people didn't just donate to the charity they liked. I mean, why run 3 miles AND pay money? Just give them money. But now I get it. This is fun. Let's do another one."