There's a half I've done a few times that has a 5K along the end of the half route that starts 30 min after the half. My half is about 1:40 as well.. you'd think by the time I got to the finish the 5K would be completely over, haha nope. One minute I'm in shitlord heaven, then suddenly BAM I'm surrounded by the fattest of the 5K fats waddling along. There are always 40-50 fats doing 20+ min pace to pass at the end. It's horrifying but also motivating, and often hilarious--hams wearing compression gear, LOL and OMG.