At 285 pounds, I can run a 12 minute mile? All other things being equal, how fast would I be able to run at 185 pounds?
You said all things being equal then the answer is 12 minutes per mile.
But I bet that wasn't the answer you were looking for. Loosing weight is just one piece of the equation. If during that time that you were loosing weight you did not run at all then your lungs and legs haven't acclimated to the lighter version of you. So their capacity to sprint and run have not improved.
Now if for e.g. you continued to run as you lost weight then things aren't equal anymore. But it still depends on how you ran. Were you doing sprints or were you just increasing your distance but kept your pace steady.
With the right training you can do 5 minute miles but with just weight loss alone it is hard to make an estimate.