Long slow runs build base fitness and stamina. There really are no short cuts. I used to hate the slow run because it felt like I wasn't putting any effort in. Then after a while, you notice a jump in distance/speed for the same time and perceived effort.
Sometimes it's good to put the numbers aside and just run on how you feel. We can all have bad days, where our HR's are all over the place, or the wind is in our faces. It can be difficult to ignore the technology we have at our disposal these days, with GPS tracking and HRM's which can all be uploaded to a fancy ap that tells us that today we ran 0.3% better than yesterday. Ron Hill didn't have all this at his disposal, and still mananged to break 2:10 for the marathon.
I would be good to do an experiment where you stick your GPS/HRM on at the start of an LSR, switch it off at the end, then upload it without looking at/analysing the numbers. You could do this over a 2 months or so of running progressively longer runs (no more than 10% distance increase per week) at level 5 or 6 (on a scale of 0 -10 perceived effort; 0 being sat on your bum doing nothing, 10 being max effort). In fact this would make a good project for someone doing a sports science degree.
Hill training develops strength and stamina. There are several ways to approach hill training. Running a hilly route is one way, letting the terrain dictate the pace. Kenyan or continuous hills is another way. Find a hill of around 5 to 10% incline (plenty in Lincolnshire) and run up and down it at a continuous pace (80% effort or there abouts) for a set time (say 5 minutes), then significantly drop the pace but keep running up and down for 5 minutes, then back to fast pace. Continue this cycle of fast and slow pace running for your work out period. 20 - 25 minutes should be enough for a first session. Once you get comfortable, you can increase the effort sections and lower the rest sections. You can also increase the overal length of the workout once you get fitter.
Canwick Hill, Cross O'cliff Hill, Station Road (Waddington), Main Street (Burton). Basically anywhere Southwest of Lincoln that straddles the Lincoln Cliff would be perfect for hill sessions. Plenty of roads, farm tracks and paths to choose from. A 1:25000 scale map of Lincon would be worth getting. You can discover some fantastic routes from checking out your local OS map.
There comes a point during a run when you may start to loose form. Normally towards the end of a long run or tempo/speedwork session. The weaker your core, the sooner the drop in form sets in. It pays to take more notice of your running form. I do an active posture reset every couple of miles or so throughout my runs. It might look a bit silly, but it's just a case of making sure I'm standing tall (not slouching), leaning (not bending) slightly forwards and relaxed at the shoulders.