Hi,
I am assuming you are 30 years old, and using MAF 180 - age formula.
Assuming you are running 5 times weekly for week volume 25 km, 1 hour each run makes it 12 min/km for 5km? Yes?
Is your pace consistent for the 5km, Is your HR consistent? If you can maintain a consistent HR... that is very good di, no matter the pace.
Say 1km, 120 bpm, 2km 140bpm, 3km 141bpm 4km 139bpm and 5km 140bpm. That is good. Normally, most people avg HR will continue to drift higher and higher as you run longer.
What improvements are you looking for? Generally, first sign is lower avg HR, then your HR will be consistent, the range may be big at start, but over time, you will find a small range. That will happen if you continue running regularly long period. Then over time, some 6 months, some 8 months, pace comes faster but HR still the same.
Some people get both pace faster and HR lower at same time. Some both at 2-3 months, 6-12 months are normal, you dun decide, body decide. So be patient.
The most important thing is keep heart rate low or/and within "aerobic threshold". Whatever you want to do, if you want to increase duration is this... you are fit for a certain point... any run beyond that, HR will start to drift, once you passed 30% HR drift, pointless to increase duration. So no point to run each run 2 hours, you will deplete muscle glycogen, and HR past a certain point, will not be consistent, it will float to LT HR. However, having said that, those with big aerobic endurance, can run 1 hour or 2 hours or 6 hours at consistent HR below MAF HR - these people take years to train.
If you want to increase duration: start with 2nd set concept ... either you wait or rest till HR recover, may be 10 mins, or reach 100bpm. Then you attempt 2nd set with same effort but watch your HR, keep HR low, it will be hard. What you will find, at same HR, it will take you much longer; but at same pace, HR will be much higher. Eventually, boths run will have a smaller variance in HR: First time say, 1st run avg HR is 140bpm, 2nd run is may be 155bpm. It will go down say 135 bpm for 1st run and 2nd run is 140bpm, which is very good di.
I tried the "duration increase", using 2nd set concept, each run is 30 mins. Each set is about 30 mins or 4K according to my fitness . Took me at least 6 months to minimize variance between 2 sets. Today i run 60 mins sometimes, not 2 set of 30 mins.
At a given certain duration, you will start with X km. Once you get fitter, at same HR, you will be able to run faster. Normal people just keep same effort: HR will drop when effort and distance is the maintained. Some keep same HR, pace will increase. Most important thing, keep HR below aerobic threshold.
You are doing 5 runs di per week, you are spending 1 hour each run. Not much parameter to tune. Not sure if you should run 6 days per week, only you know, not sure if you should do double volume ie run 2 times in a day. But whatever you do, do have a "downer week" at the start: say "3 weeks upper, 1 week downer", or/and every 10-12 weeks or 3 months deload. Body cannot take continouosly increase, it will break.
Upper and Downer week: some do 3 weeks upper 1 week downer, fitter will do 5 upper 1 downer. One example - 30km for 3 weeks 25 km for 1 week.
Maybe on weekend, say sat you do more set repeats, say 3 sets of 30 mins. It is ok to have slower and slower sets. 90 mins is a good limit to start.
Why dun i say do "speed workout" or "theshold run"? At 25 km weekly volume, it may be high for some. May be low for others. If you want to race 5k, i think 25km weekly is ok volume. If you want to race marathon, it is not enough. It depends, i took the high milage route, and build the weekly volume, i think there is a point, we will reach a plateau, then you tune again. If you do "speed workout", some just add once a week, a faster 30 mins run at below LT heart rate: others do a slightly faster last km segment of each run, but HR still below LT point.
For your info, i am not a coach, not a pro runner. But i have done some trial and error in trying out low heart rate running. This month, my monthly volume should exceed 160km, used to be 100-120 km monthly.
What I did, i ran nonstop, say 4 days continuosly, then break 1 day. Slowly i started with 5 days continously then break 1 day.
I have also tried some speed work for 2-3 weeks after 6 months. My experience, it is hard. Your body is so used to low heart rate stimuli, then you go give it a shock. Downside - why i dun recommend "speed work", your heart now will easily go above MAF HR. Point of low heart rate training - run at low HR. So those middle distance runners, they do speed workout. Anyway, middle distance runner, when they race, HR float sky high.
Actually I am just 20 years old , I heard of the MAF run which my heart rate is around 150-160.
However , I also read about Zone 2 training which said to be quite effective too at around 140bpm so I might just train from 140bpm.
However , my heart rate graph is really unstable which as you said , HR drift? I am so frustrated when I am running at the same pace at my desired heart rate and my HR suddenly spikes up. It is weird that I maintained at the same pace but the HR just gone crazy.
Of course, my goals are to run any race range from 5k to FM and to build my aerobic base too which is run fast at lower heart rate.
I will definitely look into the Upper and Downer week as mentioned by you.