In time it's possible for you to run a marathon averaging 168-170 HR the whole way (based on your 191 HRmax).
Right now you know you cannot hold that effort for that long, but it'll come with training.
Your easy running should be ~70% HRmax. So for you that means low-130s HR (say 130-135). Right now, that probably feels like walking and you don't want to go "that slow", but I would suggest you work down to it over time.
Right now you can keep your HR under 145 for a long run, once you find this easy to do (ie: the pace doesn't feel maddeningly slow) try keeping the HR under 140 for long stretches (although you can let it climb on uphills, but once on the flat again at the top, once again bring the HR under control).
Two days per week do this:
Tue: a session at 150-155 HR (~80% HRmax)
Fri: a session at 155-160 HR (~85% HRmax)
The Tues session can start at 2 x 15 mins (as you are doing) and slowly build up along these lines as you get more and more comfortable at that effort:
2 x 15 mins, 2 x 20 mins, 3 x 15 mins, 1 x 30 mins, 30 mins + 15 mins, 1 x 40 mins, 35 mins + 15 mins, 1 x 45 mins...
There is no need to take every step I give above, but slowly try and increase the amount of time you can run at 150-155. Take a relaxed 3-5 mins jog break in between each long rep.
The Fri session you can begin shorter;
2 x 8-10 mins with 3-5 mins jog break. Then build to 2 x 12 mins, 2 x 15 mins, etc. Don't be in a hurry to build, let the physical adaptations happen in your muscles and it will get easier and easier as the weeks go by.
You should find that as well as you gaining the ability to go longer and longer at this effort level, that the pace at these HRs improves over the weeks. But don't get too anal about whether each session is faster than the week before, just get the work in. Improvement is not linear.
Give it 6 weeks at these effort levels (increasing the length of the reps over the six-week period), and then post up again.
Do a 10 min jog warm up before each session. Some simple stretches and 3 x 100m strides with 60 secs jog break) and then begin.