Well a good bike shop finally moved into Daejeon and the owner, Kim Hyeong Shik, speaks English very well. He has also competed in 12 triathlons (i can't remember if it was 12 triathlons or 12 IronMans).
It's in mannyeon dong, near gang byeon apartments. if you need better directions, i can get those for you.
name: Geo Cycle - Triathlon, Road Bike, MTB, Running