It probably depends on the kid. I wouldn't have DS do it. He is good on hikes, but gets distracted easily, and it takes forever. My 7 year old and 5 year old are both running a 1k kids run on Thanksgiving. My 7 year old can run a 5k, but I selfishly want to race! It would still be a long distance for the 7 year old.
I am in the same exact situation. I usually push the kids in the double jogging stroller, but now that the older one is 5, he wants to run (I think it is because they give kids chocolate medals at the end). I'll push the 3 year old, let the 5 year-old run, and he can hop in whenever he gets too tired.
Three miles with my son on foot would be pure torture for all involved... especially the one who ends up piggybacking him. He's going to stick with kids' runs for a while, and the jogging stroller when necessary.