The one exception in training is threshold runs. I would keep those runs at an even pace, for the simple reason that it will help keep your recovery time to a minimum. That's why threshold running is so great—you get in a workout that improves your aerobic fitness, but it doesn't knock you to your knees and, thus, allows you to come back a couple of days later and do another quality workout.
Don't Blow Your Chance for a PR From the Start
Possibly the biggest mistake runners make on race day is going out too fast. Why? Because the start of a road race is thrilling. Everyone around you has nervous energy, music is blaring, and you're simply excited. But that can be a big problem—if you don't reign in your energy, you'll go out too fast for the first mile. That, coupled with the fact that 90 percent of the runners around you will also start too fast, and you have a recipe for a positive-split race, a race where your second half is slower than your first half. This is exactly what you don't want if you're trying to achieve your best possible time.
So what's the solution? You have to let everyone else go and trust that all of the work you have done at race pace has taught you what pace is right for you. If you have a GPS watch, that's obviously helpful, but it's not necessary. If you've done fartlek workouts, threshold runs and long runs, then you know how you should feel—you should feel like you're running a challenging pace, but definitely a pace you can maintain for 5K or 10K. In other words, if you're in race shape, then the first mile of a 5K or the first two miles of a 10K shouldn't be extremely hard. Yes, you'll be a bit uncomfortable, but you shouldn't be suffering.
Build Your Race Strategy
Many novice runners prefer to break the race up into pieces. For a 5K, you might plan to run comfortably for the first mile, stay steady for the second, and then run hard for the third, with a kick for the final 0.1 mile. After you've run a couple of 5Ks you can try speeding up for the first half of the final mile, and then running as hard as you can to the finish (which is just over 800m away). That takes some practice, but it's a great way to run a PR.
To convert this plan to the 10K, simply run the first four to five miles at a solid, steady pace, then run the last one to two miles hard. It pays to be cautious with your first 10K; if you go out too hard, you'll likely end up slowing down within the first 5K and end up staggering to the finish line. Go back to the principles above and, before the gun goes off, remind yourself that you can always speed up in the final mile or two, but you can't recover from going out too hard.
The reality with distance running is that, at some point, the only way to run a PR is to learn to be extremely uncomfortable at the end of the race. And after you've run a dozen or so races you will have reached a point where it is harder and harder to PR. To find that magic time, you'll have to dig deep, "go to the well," and continue to run hard while being extremely uncomfortable. That's the name of the game as you go from a novice runner to a seasoned racer—the ability to run a consistent pace, and then speed up at the end, when you're uncomfortable.
Whether you're ready to go there or not, you can still run a smart race by running negative splits. You'll leave having had a good experience, even if perhaps you could have run a little faster for your first go. That'll just be all the more motivation to PR at your next race.
More: How to Train for a PRSign up for your next 5K.