Runner who has done a marathon here, I think the wall your hitting is when your body is switching gears from warm-up to cardio. Everyone is suggesting the couch-to-5/10k programs and that may help. I would suggest pushing your time spent running a bit further, more time doing cardio should improve things.

I just restarted my long-run training for the year (because its not cold outside anymore, and I want to run a race this year, its been a bit) and just passed that point where I can just go without having to take walking breaks on my "default loop".

Also goals, having something to look forward to or to work towards helps a lot. Keep up the good work.