I suggest training more than 3 times a week, you are probably not doing enough miles to support your long run. I think the wisdom goes the long run should be about a third of the total mileage for the week or thereabouts.
My heart rate seems to go to 185 ish on a 5K race, tempo run is 165, medium pace, 150, long slow 140 ish and slow recovery 110-130. I can't be bothered with exact numbers and go more by feel. I've noticed on Strava that other folk have lower heart rates on some runs but think I'm happy at my pace. Also I drink two cups of coffee before running often! That must up the heart-rate?
Can I ask some of you more experienced folk.......I did a half marathon on 6 December....in 1.47....PB....seeing as you ask.....since then I haven't done a long run....longest was a hilly 9 miles last week. I trained well for the HM with several long runs, the longest 15 miles. After a couple of weeks of rest/lower mileage and some speedy 5Ks (for me) I am now picking up the mileage ready for another HM in early Feb and then building to a marathon in May. My question....I've eventually got there!...is....how long a long run should I/could I start off with now. Do I need to step back to 10 miles and build up again or could I jump in at 12/14/16? Thanks!