My child started running as soon as he could around 18 months when we were in public- parks, grocery stores, any store, pickup from daycare, etc. I tried ALL the things to get him to stop but none worked. When he was 3 I saw someone post in a local moms group a similar post to yours asking for advice from parents of “Reformed runners”. 8 replies and every single one said the same thing- its due to sensory processing and is flight response and to have child evaluated for Occupational therapy and it helps immensely. Light bulb went off in my head! I had my son evaluated at 3 for OT services- he qualified and has been in OT for 9 months and it has been a godsend! He rarely runs from me in public. We have learned tools for how he can calm himself down and types of sensory inputs he needs. Good luck! It will get better!