Hi-I am new-like never been on a bike (well since high school anyway ). I am an ultra runner with a heel stress fracture so I am buying a bike! I have found a cannondale:

Cannondale CAAD 8 (T6061 aluminum frame) with a Shimano Sora Triple, 50/39/30 crankset and front derailleur and Shimano Tiagra shifters, brakes, and rear derailleur

it is on craigslist they say ridden just for 1 year, not a lot---any opinions of this bike for a beginner? I am going to give a try at a half ironman in September. I have been shopping and I am trying to find the best bike I can get for $500. I understand that the 105 is more preferable. I am not looking to win-just to be comfortable and not look like an idiot.

Thanks!