Half IM run training - minutes or miles?

There are several reasons for training based on time:

1. Time is easier to schedule into a busy week of work and life.  I know I have a 1 hour run, so I schedule it in.
2. Your body knows time; it doesn't know distance.  Your body responds and adapts to how long and how intense the training sessions are.  It doesn't really know or care how far you've gone
3. Scheduling workouts based on time compensates for different workout conditions.  For, example, here in Dallas in the summer it's really hot.  If I go out for a 1 hour run in 95 degree weather, in order to stay in my HR training zones I'm going to have to slow down (compensate for the heat).  On the other hand, if I based my training on distance, a 10 mile run in 95 degree weather would put me out there much longer than a 10 mile run in 50 degree weather.  (hope this makes sense)  And again, your body knows time.  Being out there longer is a more stressful workout.

You'll have to develop some trust in your training plan.  Long slow runs will improve your speed over the long term.  I've found that improvements in performance happen in steps.  One day you are running 11:00'/mile then 2 weeks later, all of a sudden, you can run 10:30'/mile at the same HR. 

Finally, what are your goals for this race?  Is it just to finish?  Look at the time cutoffs for the event.  Can you make the time cutoff for the bike?  If so then a slow run should get you to the finish line. 

TJ