Hey guys! I started running 2 years ago and despite running at what I thought was an easy pace/low HR for my fitness - I have not seen any significant improvements in my race times over the two years, especially at the half/marathon distances.
I also and noticed a huge drop-offs in my performance as the distance increases - despite what appears to be an encouraging vo2 max test in some ways which put it at 51.5. See attached pics

I am trying to work out if I have not been running "easy" enough as I am a bit stumped otherwise...
**PBs**
5k: 20:04
10k: 46:21
Half: 1:53:09
Marathon: 4:09:19
**Background**
Male, 34, 86kg, 6'3
Always been sporty but only been running consistently 2 years
Naturally more explosive - I've always been faster at shorter distances
Max HR looks to be around 185-190
**Training**
Built up to 40 miles a week for the last six months
Running 4-5 times a week over varied distances and flat/hills
Have been running consistently at what I thought was an easy pace - 9:30 - 10 minute miles which keeps my HR around 148 - 152
However, once I get to 8:30 - 9 minute miles I am struggling to hold this pace for a long time
**Takeaways**
I always feel like my lungs/heart are fine deep into runs but it's my legs that don't have any power and start to really feel heavy
I'd been planning to drop my easy HR down to 135ish and just grind there for a few months as I thought maybe I had been running too hard? But my amateur reading of my vo2 max test results have put some doubt in my mind, as I think I should be capable of better times than this already?
Would really appreciate any tips/suggestions

Do I just have a really poor aerobic base that needs a lot more training at a truly easy pace?
Answers
I went down the rabbit hole of reading about Hadd training and am committed to running much slower now - I guess it's just humbling when you thought 8:30-9 minute miles were easy and now I am down to 11