1/2 Marathon during HIM training?

I don't see any problem with that, use the Run as a long run. Are you looking to actually compete or use it as training? If the latter have fun. However, I'd still work on building your aerobic capacity. 

Scott

Absolutely do it, just take it easy.  I'm doing about the same thing, I have a 25K(15.5 mile) run 3 weeks before my first HIM in May.  I'm going to put on my HR Strap/Monitor and keep my heart rate below 160 for the whole run.

Agree with this.  I raced a few 10-milers en route to a HIM last season, and I'll be racing a number of HIM's and half marathons this year during my build up to IMC.

I would hate to see someone hurt themselves by racing hard, and then not being able to do their HIM.  Certainly you could use this as a training run, take it easy, then run at target pace, take it easy, target pace, etc.

Well the more I look at this, I didn't realize these races were 2 months apart.  So yes go ahead and race it.

I'm in a different  boat, mine is only 3 weeks apart.  And the run I'm doing is 2.4 miles longer than a HIM run.  This is only my 2nd year of doing Tri's and will be my FIRST HIM, so that's why I'm taking it easy on my 25K run.  I just want to go out, enjoy the crowds, enjoy the run, and a 25K run is a huge accomplishment for me as well.

I agree whole heartly, Race that Sucker.

I think I am going to do it.  How should I lead up to the race? Should I continue my HIM plan and just make it my long run? Also, if I race it, do I still do my long bike the next day? Thanks.

Well, I can't answer that for you. I know, when I ran my 1/2 in February, I was only allowed to do each exercise for a 1/2 hour each each day at my recovery pace. For a total not more than 3 hours total for the week. High cadence in my opinion. 

Scott

For first IM I did a 1/2 Mary on a Sunday with no taper (PBed!) and a 100+ mile ride the next day with a short run. Later in the week I felt so fatigued I had to take a few days off of training.

You shouldn't do any other high-intensity running the week prior (save that for the race).  Depending how much you usually run, you may want to cut back your volume slightly, too.  I would do most of your planned biking, but drop most of the intensity a few days before the race.  Swimming you can probably keep largely as-is.

Doing a long bike the next day probably isn't a great idea unless you have a pretty solid base to do this from (guessing from your question that you don't).  Push the bike out a day or two, if you can.  If not, just do what you you feel comfortable with the next day.  Don't be afraid to skip it entirely.  Keep runs short & easy, if you do them, for a few days.  By mid-week, you should pretty much be back to training "normally".