It's a race.   You run as fast as you can and still cover the distance with more or less equal mile splits (wind and hills are another story).   You should be able to finish strong, but be spent at the end.

I'd say it depends on your goal.  You said you were using it as a training run, so you made the right call to take it easy and not race it.  I have a goal half next weekend, and plan on laying it all out there and being wiped out by the end of the race.

I think it's something between these two.  It sounds like you went out too fast for your first half, which I would say is different than running "hard".  I think your problem with the first race was pacing.  But, a race shouldn't feel like a training run, either.  For a half, (I think) you should feel like you gave just about everything you had.  Maybe not staggering across the finish line and collapsing but definitely feeling like you wouldn't want to run another mile.  But, the best way to get a sense for race effort is to run more races.

+1 to this comment!

It really does depend on your fitness level and how well trained you are - obviously you want to pace it out so that you use all your energy to the best of what you've got, for some individuals that means they can run "balls to the wall" and hold it for 21.1k. We're all different, focus on what you want to accomplish out of this race and give it a go!

 

To answer your question, no, you're not doing harm to your body if you race the half marathon during training for a full. You would be getting a better training effect by racing it as hard as you can, in fact. This is assuming you're not injured and it's more than 3 weeks before your marathon.

I push pretty hard, but not stupid. My last half, the pace was about 15 seconds more per mile than my tempo pace I use for 5K training. That wasn't really a spontaneous occurrence though--I carefully planned my race ahead of time. I'm ambitious, so I chose an ambitious goal. That goal (the overall finish time goal) was selected based on how I performed in my last half, and how I performed in a more recent 9-miler on a harder course. From there, I figured out the average pace I'd need to hit, and worked it out so the first few miles would be run more conservatively than the rest (there's a negative split calculator online you can Google which is helpful). Once I got to mile 10, it was pedal to the metal with all I had. When I finished, I wasn't nauseous or anything, but I had nothing left in my legs.

Even pacing is great if you are precisely correct in guessing what you are capable of that day, but if you overestimate your fitness it is a recipe for a blowup at mile 10.  I have only done 4 HM's, as opposed to the 13 marathons, but i did just go below 1:30 using a measured strategy.  I set my goal pace at 10-15 seconds slower than my 7 mile tempo run pace.  Then i started the half at 10 seconds slower than that pace for the first couple of miles.  From there, i dropped down to goal pace for the next 4 miles, then below goal pace for the next 4 miles.  I shifted into a different mindset the last 5K, just looking at it as a really fast short tempo run, dropping the pace down with each succeeding mile until finishing the last mile at 10K race pace.  You give up 20-30 seconds total those first 4-5 miles, but the race is long enough to not only make it up, but get below your goal time while finishing strong, passing people.  Finishing strong is a real confidence booster no matter what your final time was.

This is all decent advice if you're main goal is to avoid a blow up and have a confidence boosting experience, but if you want to see what you're capable of, occasionally you have to risk blowing up. You have to take into account the course profile, your competition, whether the  mental let down of letting a group or a competitor go is better than trying to hang and risking a blow up, etc. There are lots of little things that come into play.

My last half marathon (last Sunday) I was probably a tiny bit too aggressive in miles 5-7, but I was running stride for stride with one of my  buddies/teammates/competitors and we were in a good groove, working well off of each other and neither of us wanted to blink. I wound up fading quite a bit after mile 10, where he didn't. He got me this time, but maybe next time I'll get him. That's racing. I still hung on for a better finish than I probably would have if I'd set a conservative goal and tried to negative split the race. At age 43 it wound up being my 2nd or 3rd fastest half marathon ever and I've run a bunch of them. No regrets.

mikey and Jeff are spot on. I'm still trying to get into shape but I pushed the pedal hard yesterday and worked with another guy for 9 miles. We both surprised ourselves with times a couple of minutes under our goal  for the day but neither of us had a "what if". In our case we didn't fade but managed to pick it up and work through the pain. Good physical and mental confidence builder for the next phase and goal HM in a couple of months.

To your question, unless your a few weeks out from a goal marathon shove the pace as hard as you can. You will learn much about yourself and your running and if you blow up so be it. You can turn it into a nice long run and not do any damage. If you don't know where the red line is, you won't know how to ride along it when you really want to chase that PR.

BTW - I'm talking more about feel here. I ran without fiddling with my timex and was focused on how it felt to race hard. The other fellow had the garmin and was watching splits but I was focused on the intensity and hurt. That's the only way I can move past that level of discomfort to new levels. But then again, I'm just a hobby jogger at heart.