Hey, Runatic!
parkrun has a volunteer role called Tail Walker.
This person stays behind the participants and completes the course alongside those moving slowest. The role is required at every 5 km parkrun and 2 km junior parkrun. Volunteer roles
You can walk an ordinary 5 km parkrun. The Tail Walker receives both volunteer credit and a recorded finish.
Worrying about coming last becomes rather interesting when that position is already reserved by someone who came specifically for it.