My daughter and I would feel weird at this point to run a 10K or half-marathon without costumes. For the Marvel #DoctorStrange10K today at Disneyland, she made a cute Thor costume and I made an Ironman outfit. Actually I did not bother very much with the rest of the costume details because I spent all my time geeking out over making the lighted arc reactor. Eventually, I ended up with a very light design using an electro-luminescent panel with an inkjet-printed overlay on transparency film. I had to hump the battery pack over the whole course but it was not too bad.
Update: Per several suggestions, a pocket in the shirt for a smartphone with the image of the arc reactor on the screen does not work well. The phone is too heavy and unless the shirt is really, really tight, it flops around when running. The EL panel is super light, and the main weight of the battery can be on the belt.