I've never done a tri (I'll do my first this year!) but when I've been training for half marathon events and I have more weeks before the event than the training plan, then I just stretch it out.

Right now i"ve got a plan that's 17 weeks, and I'm 20 weeks out. So I added in a couple more short runs at the beginning, I repeated a week or two, etc. Just stretching it out like that where it seemed it to make sense.