As you do more training and more races at different distances, you should begin to know your goal paces like the back of your hand. If you are early in your training, you can focus on perceived effort below and use Hanson's handy calculator.
The main ones to focus on are:
1) Marathon pace: 65-70% perceived effort. A pace you can sustain for a few hours.
2) Half-Marathon pace: 80% effort. A pace where you start to feel comfortably uncomfortable.
3) 10k pace: 85% effort. A pace where you are short of breath and can't hold a conversation.
4) 5k pace: 90% effort. A pace you can sustain for less than 30 minutes and will be out of breath by the end.
While most of your training may be done at an easy run pace, often slower than your marathon pace, or at your marathon and half-marathon paces, one of the best ways to become a faster runner is to train at faster paces.
Lactate threshold workouts tend to be in between your half-marathon and 10k paces, where lactate accumulates in the blood at a faster rate than it can be removed. Improving your LT allows you to run faster for a longer time, before lactate levels become intolerable resulting in physical fatigue. This can be achieved by longer, Tempo runs, which take place at a pace faster than your Half-Marathon but just slower than your 10k pace.
Your VO2 max, the maximum amount of oxygen that you can use during running, can be expanded by more running at or faster than your 5k pace for longer. While it's hard to be training for a 5k and a marathon at the same time, mix in the occasional shorter distance races in your training or maintain consistent speed work at 5k and 10k paces to improve your LT and VO2 max!