I couldn't even run for five minutes straight! How should I start training again?

I used to run about three to five miles a day and then my treadmill broke. (about four months ago) so I stopped running since it was snowy and cold out. I finally got new running shoes so I took a "run" yesterday. I ran about three blocks and I was out of breathe! How do I get back to where I used to? I have asthma so running never came easy. How can I build up my strength and endurance again?