Your 10k of 38.00 (6.07m/m) could suggest that an easy run pace of 7.45-8.00m/m is fine, but your marathon time of 3.15 (7.26m/m) puts a different slant on things.
I could enquire whether your marathon was a solid effort with no ?crashing?, which would explain the relatively slow time. Instead I will assume you did not crash and walk at any point, but that it was a steady run effort the whole way and the slow(ish) pace was simply due to a lack of adequate preparation (meaning, in your legs).
I coached one female friend to a 3.15 in London and know for sure that she has never broken 42.00 for 10k, so your 3.15 is somewhat ?soft?, but a pretty good effort after only one year of training.
Your 38/10k suggests a 2.55 sort of marathon time as a more solid performance, but since you feel your 10k can improve you are right to concentrate on it first and knock a huge chunk off the marathon in a year or so.
My 3.15 friend runs easy at 135-145 HR, which for her means approx: 8.15-8.40m/m. When I do not know anything about the runner, I tend to suggest a safe easy pace of marathon pace per mile (M-pace) plus 1.00-1.15 per mile. For you that would mean 8.25-8.40. Just as for my friend, since your M-times are identical.
Now you might find that WAAAAAYYYYYYY slow at first and doubt my advice, but I KNOW her runs are easy, because I also know her HR while doing them. I?ve also run 2+ hours with her at this pace and chatted to her the whole time.
Can you do as I recommended to RunHard and beg, borrow or steal a HR monitor? Even for a little while, like 60 mins? Check yr HR on your easy pace, and then check your pace at approx 140 HR. I am hoping they are very close, but I suspect they are not. Even stop and take your pulse at your neck (get used to finding it, before starting to run). Take it for 6 secs and multiply by 10 for beats per minute.
Do not assume that simply because you are able to run at a certain pace, that that pace is having a positive effect on you. And do not assume that because you are training for a 10k that you can run faster on your easy days than you would if you were training for a marathon.
You appear to be learning from your reading and you should continue to read as much as possible. You can always raise questions, to which you can find no answers, on this forum.
Very well 10k-trained runners can also run very good marathons (M-pace per mile = 10k-pace + 30secs mile). Some even go so far as to say that marathon training is simply 10k training plus long runs. This is an over-simplification, but you get the idea that if the distances are linked, so must be the training.
Now this apparently slow pace I am advising can improve with training. The pace can improve while the effort remains the same. Which explains why Paula Radcliffe can clip along at 6.00m/m while her M-pace is only 5.16m/m (so she is not running M-pace + 1.00-1.15 slower like I advise). But I would not expect you to be as well trained as she is, yet.
The short answer? Check yr HR on your easy runs. I would expect that anything over 145 HR is too high for you at present, although it might feel ridiculously slow. In fact, the slower it seems to be, the more you need to train at that HR. In time, the speed will improve (within weeks).
If you can, keep to this HR and go further/longer. Much longer. Semi-regular Sunday runs of 2 hrs+ will do wonders for your 10k time (always take adequate recovery afterwards and eat and drink well before running long or hard again).
If you are unable/unwilling to check yr HR but feel confident with your current easy pace. I suggest that at least you run slower on your Sundays and go a lot longer than at present.