U+ is like the Sprint of Korea. It's not nearly as good as KT or SK. Everyone with the Egg has always had good results and a strong charge.
And yes, two weeks does add up. One thing you could do is get an Olleh WiFi unlimited card for three days. It's 9,000₩ and they have them at all convenience stores. Just log in with that card at an olleh Hotspot. That could hold you over until the end of the 3 day period. Olleh has a MASSIVE wifi network, way ahead of any other carrier. Of course, subways, but also bus stops, Starbucks, doctor's offices. You name it, they're there.