Donovan - you're probably right at this point in Dave's development,
and that he's only 5 weeks away from his event (I know, because I'm
targetting the marathon at Gold Coast on the same day).
In the medium term, speed work has huge benefits for distance runners.
Work on speed specific stuff (400 and 800 reps, intervals of 1k into
pyramid sessions) has enormous benefit for distance runners in terms of
cardio fitness and technique efficiency gains.
I've been able to shift from a mid 38 10k runner to 35-36, and seen my
half marathon time drop from mid 80s to sub 80. I'm aiming at 2:47 at
Gold Coast. The big gains have come from two interval sessions per
week (one short intervals, the other longer) plus a good hard tempo run
or XC race, as well as the traditional long steady Sunday run.
Dave - at this stage, the more you run, the better you'll get. Once
you get through this, you can start to look at programs involving more
speedwork.
Personally, I think the half marathon is a great distance, particularly
since you can actually do the distance in training without wrecking
yourself for a month. Good on you, and good luck on the first Sunday
on July!