Ok, I'll add my 2 cents here. Get a fixie with a flip flop hub that takes a freewheel. When riding around town use the free hub, if you want to train use the fixed side. I am not anywhere near triathlete condition and I can make it up any hill I need to with a 42x16 setup. I find this ratio is fast enough that I don't feel like a turtle and low enough I can cart me up the hills with out undue strain.

I have a marathon runner friend who uses a fixie to help with his marathons, I don't see how it could be anything but beneficial to your training.