Get your base mileage up.
I started running in late 2012, completed 1/2 1/2 marathon in May 2013 and first full marathon in may 2014.
Having been a couch potato for over 30 years, I could not even run one lap at beginning.
Every time I try to get up speed by running fast, I would get injured. Then I just start adding base mileage. Once you get up to 50 ~ 60 miles a week, you should be able to run much faster for a mile or two.