How many miles riding = miles running?

That is my wacky HR profile for my 5.5 mile/2o minute bike commute into work yesterday. First, my max HR is only about 175, and that big peak at the beginning is a wierd artifact so ignore it. Most surprising to me was the average HR of 120 over the whole ride (about 60% of my heart rate reserve). I have to run really slowly to keep it at 120. I was riding reasonably hard during this ride (I never stop pedaling when I ride, except when I'm stopped at a traffic light), enough that my thighs were feeling it.

My run at twin brook tuesday night was at a 9:20/mile pace and my average HR was 129 (about 65% of heart rate reserve). Let's say, then that a 10:00 min/mile pace would keep me at 120 b/m, equal to the bike HR. So I'm burning the same amount of aeobic calories on my bike at a pace of 16.6 mph as running at 10 min/mile. Interesting. Anyway, this means that I would need to go 16.6 miles (what I'd do in 1 hour on the bike) for every 6 miles of run (what I'd do in the run) to use the same amount of energy (or burn the same number of calories). That's almost 3X. I'll be able to nail that number down a little better when I get more HR data but its a pretty good comparison to how the professionals have measured bike miles to run miles conversions.