Why can't I run as far as I used to?

This time last year I was running 4 miles a day in about 27 minutes. I could comfortably run 2 miles in 12-13 minutes. About a month ago I went on my honeymoon with my new wife. Obviously I didn't do too much running then. Since I got back I have been running everyday however, I can barely make it 2 miles. My legs have really been bothering me so I figured I needed a new pair of running shoes. I went out and had my feet fitted etc. and learned my ankles roll in slightly when I run. So, the guy recommended a couple different pairs that will help with that. My legs don't hurt as bad when I run, however, my lungs don't want to go much passed a mile and half. I can barely make it 2 miles now. It's really frustrating to me because less than a year ago I was running 3-4 miles with no issues.

I went through the police academy and I am now working as a cop on a college campus. Over the last couple of weeks I have noticed while doing foot patrols that my legs are getting extremely tired when I just walk around. They don't hurt or anything. They just feel completely gassed and all I want to do is sit.

I'm thinking about making a doctor's appointment this week to make sure there isn't anything medically wrong with me, however, I'm pretty sure there's not. Any other ideas of what's going on with me?