You'll probably get a lot of answers since it varies a lot from person to person (we're all an experiment of one), but in general I'd cut it to maybe 1/2 then 1/3 of your longest run. You don't gain much (if anything) from your long runs in the last couple weeks, so no sense having it potentially harm your race. I tend to keep my long runs pretty long, but the pace is extremely mellow... conversational the entire day.

I'll normally follow a similar taper to what you're planning (2 long run weekends that are shorter than normal). Might look something like this for my long runs (this distances will vary depending on what your typical long run is, these are just examples):

Week 3: (full long run) 20-40 miles
Week 2: (1/2 distance) 10-15 miles
Week 1: (1/3 distance) 5-10 miles
Week 0: Race
Week +1: no long run
Week +2: 1/2 distance long
Week +3: normal schedule

Sometimes I can sneak back up to normal at week +2, sometimes not... depends on the race and how injured it left me.

The Indy half is a really great race, you'll have a lot of fun!

Good luck!
-Chris

As cgerber indicated, you'll get lots of answers. And, I'm probably at the far end of the spectrum. For races HM or shorter, I don't really taper, except for a mini-taper in the week before. I'll do about 80% of the miles I would normally run. Typically, I would run 40 miles. The week leading up to a HM, I might do 32. If the race is on Sunday, I'll run maybe 5 miles on Saturday (I usually rest on Friday). The week before, my long run would be 8, the week before that 10.

There are training plans on the 500Festival.com site. You might look at those and see how the tapers look to you there. They have several plans: walk, walk-run, run-walk, run, and advanced run. They have been provided by Jenny Hadfield in the past, and I expect they will be this year, too. The plans should have started last week.