Posted: 27/02/2014 at 21:59
Gord, don't worry too much about the structure of the intervals, one of the most popular formats is 'fartleks' which are unstructured sprints (meaning that you just set a target and run fast to it rather than measuring set distances). It's the combination of strength building in the longer runs and the faster interval training that will really bring you only quickly. You'll probably also find that you'll run much faster on the day due to the adrenaline/ excitement/ fear etc
In fairness and not wishing to be insulting, 14 minutes isn't that fast, so I figure that if you're fit then you'll have the potential to really eat into that. Unfortunately 3 weeks isn't that long a time.
If you do want some structure then my suggestions are to do alternative reasonably long runs (no less than 10km) with speed sessions. I'd typically do one speed session with shorter distance sprints (only a few hundred yards) at 95 -100% effort followed by a couple of minutes at a slower pace and then another speed session with 1/2 mile intervals. Rest for 5 mins then run back. Repeat as often as you can (6 to 8 times should do it) . I'm not sure that I'd even bother running the full 1.5 miles until the big day.
So day 1 = 10km; day 2 = short intervals; day 3 = 10km; day 4 = long intervals etc.