Hi @teddiemao - It sounds like you have been running for a while and are now experiencing some issues being able to run the way you have in the past. While it is expected that your pace as you age may increase I have friends in their late 60s and 70s who continue to run regularly and while their paces have slowed a little they still make me envious. 

So if you have been running a while and now are having issues completing even a portion of your normal distance, I would first get a check up with your doctor, explain what you are experiencing and make sure there is no underlying heart or other issue. I knew I had a very low HR and that it would be a problem eventually. I started having symptoms similar to yours where I couldn't run far at all because my heart couldn't keep up with what was being demanded on a run. I had a pacemaker implanted last year and now I am back running normally again (note I am 65).

If there are no issues I would go back to your normal base building plan and/or maybe try a run/walk method (Jeff Galloway has a lot about this).

Also make sure you are getting plenty of rest. As I have gotten older I have found I can't run everyday the way I used to, so I take an active rest day between runs and do other training. And after a HM or other race it also takes me longer to recover.