Tri run times vs. stand-alone run times

Depends on how much you leave on the bike.

It also depends on your training cycle. I had a half marathon in April, which was a B race for me. I PR'd the race, but I was not peaking for it.  I completed a HIM in June, which was an A race, and knocked 40 seconds off my previous half mary PR.  So your training plan has a lot to do with it, as well.

I would agree if that would have happened over a period of a year or longer. If you PR on your HIM after a few months in which you ran a stand alone ½ mary, IMO you ran the half mary way to conservative to your running capabilities. Maybe Mike Ricci can chime and tell us what he thinks…

Another factor is where you are in your training carreer.  The "younger" you are, the more quickly you will make performance jumps.

After a few years of consistent training, you should see less drastic changes from year to year than you see in your first couple years of training.

If you've only been training a year or so, a couple months between a standalone running race and a triathlon is enough time to add to your fitness significantly enough to have a faster run during the tri. 

Hopefully this will be more helpful than hijack, but based on my perceived exertion and HR, I ran the April race to the best of my ability.  My training strategy for that race was to cycle the running separately from swimming/biking/weights.  In other words, I scaled back my running workload the week prior to the race, but maintained the same workload in the other three areas (Build 1).  I believe residual fatigue from a semi-aggressive workout schedule limited my ability to run faster. But I worked hard all winter on running, so I was still faster than I was at the time of my last 13.1 in Sept, 05.  For the HIM, my training since January was focused on this race and I tapered for two weeks. Everything came together for me during that race.

that's a great improvement nevertheless. Good job!  Still I think with a few days of rest prior your 1/2 mary you could have run faster or if you do one soon you’ll crush your stand alone PR )

I was thinking about this driving into work, and had to laugh at the one detail that is taken for granted when comparing races and results - how well are the courses measured?!