That's cool! I've not seen any Simbas, but I have seen some Ariels out on the road that you could probably Google up pics for some ideas on how you might do it. Simplest is some kind of green running skirt or shorts combined with a purple tank or sports bra. I've seen some people combine the top with flesh toned material under, though I don't know where that comes from, or if those were maybe part of an actual retail costume. Some simple additions to cap off the clothing might be some kind of temporary washable hair coloring spray/chalk and a tropical flower or "starfish" hair clip. Try a Google image search for
"little mermaid running costume" for additional ideas, and maybe check out
Team Sparkle's running costumes page for ideas of how to use different materials and objects for this or other, future runs.
Simba could be a bit trickier, but maybe some clothing in a muted yellow or tan if you can find it. With all the crafty people on Etsy or possibly even in your local craft stores, you might be able to craft a headband with some lion ears and add some additional material to simulate a mane. . .
A craftier person might have an idea of what kind of materials you could use to fashion a small tail, and if you could sew it to a clip of some sort, clip it to the back of some shorts -- or like we did for our duck tails as Donald and Daisy, we had ostrich feathers on a hair clip and then safety pinned the hairclip to the back of our shorts.
Hopefully this helps give you some ideas on where you could start with these or any other costumes between now and the race!
One thing to keep in mind with costumes, though, is to try them out on a test run prior to the race to be sure that elements of the costume don't irritate you! If you aren't able to get something together before this race, just enjoy those around you and soak up the ideas for your next Disney race!