The MAF formula really is bullshit though. Anything using an arbitrary number based on age is by default going to be bollocks. That's not to say that people can't get great results - they can, but only if - A) Their HR happens to fall into a range that the formula makes sense for. B) They have a massive amount of spare time required to make low intensity exercise make sense.
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.