“Is there a difference in runng a HIM half marathon run and a stand alone?”
Yes and No?..
Essentially they are the same in that the distance is long enough that if you just try to hammer it you are going to go out too fast. A successful race is predicated by knowing what pace you need to run the 1/3 to 1/2 of the 13.1 at. The way that you do that before the race is by dialing in your tempo pace. One of your workouts ought to be something like 2x20’(3’ easy)@Tempo. This is “comfortably hard”, or the pace you could sustain for a 1 hour race. If in the last 5’ of the 2nd interval you are able to drop your pace by more than ~:05/mile, you should adjust your Tempo Pace to a faster pace for the next week’s workout. If in the final 5’ of the 2nd interval you struggle to hold the pace, you should adjust it slower. Remember, this isn’t the fastest that you can run the 2x20’, but ~95% of that. Get this pace dialed in! It will likely take you a few weeks.
For a ~2 hr runner such as yourself, you would be shooting for an open HM pace of roughly :15/mile slower than your T pace. For faster runners, they would be running closer to their T pace…a 1:30 hm runner would probably be closer to :08-:10/mile slower than T pace. Race experience will allow each individual to dial in their specifics, but these are good guidelines. I use this as a cap for the first ~4 miles. Invariably, it feels too slow and I have to force myself to slow down. At 4 miles, the early adrenaline has worn off, and I generally have a good idea of my prospects for picking up the pace if I’m feeling strong.
In a HIM, your pace will generally be similar to your open marathon pace, which is generally ballparked to :15/mile slower than your open half marathon pace. When you come out of T2 after having been going ~20 mph for a long time, this generally feels way too easy. People will be tearing past you, and it takes real discipline to stay at your goal pace for the first 3-4 miles since it feels so easy. You have to trust that if you really were able to run faster, you’d have been able to do it during your tempo runs in training. Because this pace feels easy, you ought to be able to get some nutrition in during these first few miles. At some point, someone is going to put a piano on your back, and maintaining this previously easy pace becomes very hard, but if you do, you will start reeling in many of the people who went tearing past in the early miles.
Good luck! I generally manage to even split my HIM runs, and I find it very energizing when in the last 4 miles I am picking people in front off and running them down. Usually, my HIM split is around 5 minutes slower than my open hm time.
Can’t stress enough about how important it is to both have your tempo pace dialed in as well as to do workouts at this pace each week when you are training for open halfs and HIMs. This pace gives you a lot of bang for not a lot of recovery cost or injury risk compared to shorter/faster speedwork.