I found my speed improved over time without me needing to focus much on it. I started out well over 6min/km and breaking 2 hours for a HM was hard work (that was about 5:40/km pace) but now I run at about 4:45/km as my steady pace and managed 4:30/km for the HM last weekend (that was a real grind though).
What helped me the most recently was turning off the timing alerts on my iPhone so I only got told the distance not my pace. That meant I ran based on how I felt rather than constantly worrying I was going too fast/slow compared to normal.
It means some days I get out and run quicker because I have more energy whilst other days I just enjoy a slower run. Most days I just run at the same effort level and since I am getting fitter and more efficient at running I should gradually get quicker
As for your build, at the HM on Sunday I was beaten by several people who were a lot bigger than me (and beat a lot of people who were much skinnier) so don't worry too much. Build might limit you from reaching the really quick times but you've got plenty of scope to get quicker if you want to.
What helped me the most recently was turning off the timing alerts on my iPhone so I only got told the distance not my pace. That meant I ran based on how I felt rather than constantly worrying I was going too fast/slow compared to normal.
It means some days I get out and run quicker because I have more energy whilst other days I just enjoy a slower run. Most days I just run at the same effort level and since I am getting fitter and more efficient at running I should gradually get quicker
As for your build, at the HM on Sunday I was beaten by several people who were a lot bigger than me (and beat a lot of people who were much skinnier) so don't worry too much. Build might limit you from reaching the really quick times but you've got plenty of scope to get quicker if you want to.