I think you might be underestimating how hard your 18 minute target might be to achieve. I'm a very mediocre runner, but could pass that 1.5 mile test, but I'd never ever ever ever be able to do an 18 minute 5K.
You don't train to your current fitness though, do you? Because if all you ever did was train to your current pace you wouldn't get anywhere. When doing intervals you train to a higher pace and the recovery period allows it. .
Of course you train to your current fitness!! Your training advice to be able to do a sub 40 10k involves doing tempo runs at 6:27-6:42 min/mile. So according to your plan all you need to do is knock out a tempo 10K in training in 39 minutes and 20 seconds and you should be able to manage a sub 40 10K in a race. Genius.
"39:59 for 10k is 6:26 min/miling for just over 6 miles; I think most adult men of a healthy weight should be able to run that with a good base of fitness and 20 miles a week of quality running, even if they had the worst genetics. "
Looking at the results for our local club midweek leauge on a fast course. 89 out of 423 managed sub 40. I suspect if most men could achieve that time off 20 miles of quality running, the figure would be at least double that.
And if the OP is currently doing "44:18", Easy runs: 7:16-8:15 min/mile are not going to be 'easy', bearing in mind his current race pace is 7:15...