Running fast IS supportive evidence of doping. Not that you can draw conclusions from fast times alone, but it is still evidence. For instance, if she'd run 61 minutes for the 1/2 marathon, and did nothing else "suspicious" other than that, then I think for most people, that time in itself would be pretty convincing evidence of doping.
If you accept that 61 minutes for a woman would be suspicious, then you have to ask yourself where you personally draw the line. You're OK with 65 minutes, but what about 64, 63, 62?
Look at cycling. Cycling is convenient because we KNOW that most of the riders of the recent past were doping. We know that Marco Pantani was powered by EPO when he set the Alpe d'Huez ascent record of 37 minutes. So if somebody comes out and beats that time, their performance alone constitutes evidence of doping.