OP,
Be careful with some of the advice you've received so far. Like "you need to train faster"....
Here are several thoughts, and some additional questions, after reading what you've written:
When did you run the 42:56? And, when did you run the 74 minute 10-miler?
1) "7 years on & off" - depending on what this actually looked like over the 7 year period, this could explain a lot of your "lack of improvement" (of only 1 minute) over the 5 years for the 10-mile race.
Long-term improvement is very difficult if you are regularly taking off big chunks of time. You have to start from stratch and rebuild much of what you lost.
Consistency over years, even if it is a bit lighter workload overall, works wonders and is the KEY. It took me years to figure this out. I would train HARD for 3-6 months, then get injured/burned out/"busy with life" and miss the next 3-9 months. Didn't see much improvement over the 5 years of doing that. If you put together 2-3 years of consistent training, you will be amazed at how much you improve.
2) Compared to your 10k race (42:56), that 10-miler was a terrible race for you. Based on that 10k time, your 10-miler should have been more like 1:11:11 (see calculator here:
http://www.runbayou.com/jackd.htm). You might want to hit the "+" button on the calcultor after inputting your 42:56 10k time, since it rounds down. Your are very close to the 42:50, 48 VDOT level, (whenever you ran that 10k)
Would you feel happier with your improvement if you'd run 71min?
You might evaluate why you underperformed so much in that race. I don't know, and will probably know more when you answer #9 below about your workout paces), but taking the entire week off before a key race is not an ideal taper. That would make me terribly flat, and I would run horribly after doing that. Also, if you think you "could be overtrained", then I'd guess you might be. (although my HR would be sky high on the first day back after a week off, so maybe not if that's the only thing suggesting overtraining)
3) You HAVE improved a TON if you've dropped your everyday training pace from 10:30 to 9:00/mile at the same heart rate. It's hard to tell from what you wrote - how long exactly did it take for that improvement? Was it since the start of your "base" in june/july, after the base was finished, or since Feb?
Either way, that is HUGE improvement. Don't lose sight of that. Your training IS WORKING. The only question is if it is solid, or massive improvement. You have to be very careful with your starting points when measuring "improvement".
4) If you just finished a base phase in july, that means you've only had about 3 weeks of faster running/workouts? That is not enough to expect to race to your potential, especially if the base was all 70% running with no tempo runs (not saying that's a bad way to do base, but you can't expect to race great off it without a few months of quality workouts after). When exactly was your 10-mile race?
5) Heart rate monitor - don't ditch it. It is a very helpful tool, and is getting you in the right ballpark for your training paces. (see #6)
6) Everyday training pace - your 9:00 pace is not too slow in my opinion. Or in Jack Daniels' opinion. Check out the suggested easy pace on the same calculator:
http://www.runbayou.com/jackd.htm. Your 42:56 10k (if you're in that shape now) suggests a maximum pace of about 8:50, and your 10-mile race suggests more like a 9:08. Your 9:00 pace looks about perfect to me.
7) Weight - huge factor. How does your current weight compare to the previous years you ran the 10-miler? 2 seconds per pound per mile is a decent rule of thumb.
8) Age - minor factor, but a factor. Your 1:14:00 10-mile at age 39 is about the age-equivalent of having run 1:11:45 5 years ago at age 34.
If you are relatively undertrained, then you should be able to improve faster then you are sliding down the "age hill". I have, and I'm about your age. But we are definitely fighting a little against time more than the younger guys. Not worth dwelling on, but it is a minor factor.
9) Your training - what kind of paces/times are you running in your two weekly workouts? The 400s, and the mile repeats? Once I see those, I will probably be able to give you some specific suggestions on your training.