Base Training?

New here and really need some advice.  First, some bacground. I haven't competed in four years.  I have been running about 3 days a week 3-5 miles on avg for those four years between deployments. Prior to the last four years, I was doing about four sprints and 1-2 olympic tri's each season and was training for an iron man as well as numerous 5 and 10K's.  I have started a 4 week base training phase to begin training for an olympic tri in early Oct.  For the last week I have been running with my heart rate below 150bpm for about an hour or longer. 1 day of slow intervals140-155 bpm, a few days of 140 bpm for 1 hour etc. HERE IS MY PROBLEM. I ran today for 1 hour at 150 bpm. I went 5 miles. That is a 12:00 mile. I have to run that slow to keep my heart rate below 150.  For the days when I stay below 140, it's almost walking. A VERY slow shuffle.  I am making sure to maximize steps per min(84-92) as well.  I can do 5 miles easily with my heart rate at about 185 the entire way. If I do a normal casual run, about a 9:00 mile, my heart rate is around 189 or so. It is not un-heard of to reach 190.  What is the deal?  Is 3-4 weeks of base going to make a difference at all? I welcome all comments, any points of vew etc.

Thanks in advance!