Don't think in terms of speed or distance, think in terms of time and intensity of your heart rate. If you ride your bike for one hour consistently around 160bpm, you will get a similar level of workout as if you were to run for one hour also consistently around 160bpm. Do both and see how far you biked and ran and you will have your answer.
In winter when I use the stationary bikes at the gym they say I am burning 450-530 calories an hour, iirc. That is with a constant cadence, so I would assume it would be about the same as riding a fixed gear. I have no idea what the numbers are for running, but like was previously mentioned it obviously depends on your intensity.
However this calorie calculator says that I'd be burning like 900 calories an hour, which I don't think is right. But it's top allowable speed is 17.5 mph.
I have always wondered how they equate. I alternate days of running and riding fixed to work. It's 8 miles and riding usually takes 25 minutes with running taking 70-75 minutes. By that time differential, would it be 1 mile run equals 3 miles bike, assuming you do them both at the same effort level? I have no idea. I try to do both in the same cardio zone but I'm generally a faster cyclist than I am runner. Running takes significantly more out of me.