To a certain extent you will get "faster" as you get fitter and as you say you are still early on in your running journey.
But ultimately the way to get faster is to run faster! I would start on one of your runs a week puting in a few 100 m sections at a faster pace. Doesnt have to be drop dead pace just noticably faster than your normal run. Do that for a couple of weeks then make the section a bit longer 150-200 ms.
Speed will come. Five and a bit miles in an hour is not painfully slow either
Interesting splits. Certainly you seem to have set out at a reasonable pace for a sub 4 attempt. Which means that you were undone partly perhaps by the weather (although it wasnt killer warm) and partly by your training.
You ran the last 8 miles at around 12 minute mile pace on average.
Would certainly go back and look at your training. What sort of pace did you run teh LSRs at? Have you ever used a HRM to monitor your training?
Int 1 & 2 would both have safe sort of effect as a training plan if you do the sessions at the correct pace.