SI Half Fears

I will be run / walking due to a nagging injury. I am going to be out in the rain and cold for hours. This event has a 16 minute mile cutoff, and I will probably be doing 12 minute miles. Its not like I can carry an umbrella and wear a gortex jacket. The portion that I run will still get me sweating, and I cannot wear an excessive amount of clothing. This is literally hypothermia weather and the back of the pack is going to be exposed to it for hours. What the heck do I wear / carry with me? Please do not tell me to DNS.