A year ago I was running sub 25 in 5K. I ran 4-5 days a week with a long day of 5-7 miles on Sunday. I had some health issues (not physical) so I did not run in Nov, Dec, or Jan. I started back in Feb and even tho 2 months later I can't run 2 miles at a 6mph pace. My breathing and heart are great. My feet feel great b/c I have brand new shoes. The problem is that my legs just physically quit on me. If I am on a treadmill I have to hold on for fear of falling about a mile in to the run. I'm a fairly new runner and have no idea what this may be. Any suggestions would be greatly appreciated.
Basically you're starting over. You took 3 months off and pretty much lost most of your conditioning. Mentally you remember how to do it. Physically it will usually take at least as long as the time you took off. If you did something like cycling or elliptical during those 3 months that would help to reduce your return time. At any rate, it's almost (but not quite) like coming off the couch.
What Len says is true - it can take a while to "restart". But you also note that you've been back at it for 2 months... So perhaps you may be trying to be too agressive in your re-conditioning? You don't mention what it was that sidelined you, but lets leave that alone for now...
You mention having difficulty maintaining a 6mph pace (10 min/mile) for 2 miles, so to build up your endurance, how about concentrating on a slower pace and shorter distances, and then incrementing your pace interspersed with a few 30 second walk breaks until you get to a distance level that you are comfortble running (say 3 miles) at a pace that is comfortable and fun for you?
Depending on how often you run and your overall conditioning, you should see improvement over the course of a few weeks. But as Len said, it takes patience... I've been there and I get it.