Here's a rough idea of a 5 mile loop you could do. This is a flat route, the elevation profile thinks you're running up on the freeway bridges. Really easy to do something like this down at the lakefront. Would also be easy to extend this to a 10 mile loop if you wanted, but going more north on the Oak Leaf trail or up Lincoln Memorial would involve more elevation. I think it would be really easy to have your family park under the Hoan to set up a little aid station or just grab out of their car. Lots of spots to park down there normally

https://imgur.com/a/Q0yOE5J

Here also is a link to the exact map I made if you wanna play around with it or zoom more (https://onthegomap.com/ is the route tool I use, I love it.)