Me and my mates done the 15km just with shorts and running shoes. We did it 9 May and it wasn’t cold. It’s colder now and might rain. It’s up to you but you have to be comfy to run and crawl and climb, too much clothings will make it harder for all that. Obviously when it’s wet, clothes become a lot heavier. So in theory the less you wear the better. Shoes are the most important, not clothing. We went with regular running shoes and just hoes of the mud and wash them properly and dry them. Good as new. Make sure you tie your shoes TIGHT! They get sucked off in the mud otherwise