IMO, rules of thumb are too general, I've seen anywhere from 3-4 miles of biking to a mile of running. If you're serious about training and keeping your aerobic fitness, there is no equivalent mile on a bike versus running, you should measure your heart rate and try to approach your running heart rate. Ii will be difficult because your not bike fit. Speed is everything on a bike, because the main cause of work is air resistance, including wind. I like to think of 15mph on a bike like an 9 to 10 min per mile jog and you have to approach 20mph to get down to 8 min miles but it's not linear because air resistance is exponential.
This table will give some idea work based on calories burned.