Different events can require completely different mindsets about swimming, and in the same way that you would never swim butterfly in a breaststroke race, you should never approach a distance event the same way you would a sprint. The 500 free, while technically more of a middle distance, is generally the gateway event into longer distances for many swimmers and is really the first event where being able to hold a pace is necessary to do well.
It is also important to note that in longer events especially, good turns are just as important as good swimming. One extra dolphin kick off of each wall may not make much difference in a 100 free, but in a 500 where there are 19 turns, the difference can be quite noticeable.
Here are a few different approaches to the 500!
Consistent Pace
The first 50 will always tend to be the fastest just by virtue of having a dive. Try to ease back a bit for the first 100, as this will be when it feels easiest to go a faster pace than you can hold.
If trying to hold a specific pace, it can sometimes be helpful to watch someone in the lane next to you, assuming they are holding a similar speed. Although doing this is very situational, and isn’t a reliable way to hold pace.
Over the course of the swim it is going to take more effort to go the same speed as you get tired. Increasing stroke rate, and/or kick over the course of the race can help minimize any slowdown. Try to go slightly faster each 100 to maintain your speed.
In the last 50 give anything you have left to power into the finish.
Negative Splitting
In the first 50 it is important to establish your positioning in the heat, if you fall behind this early, it can be difficult to catch up and use the people around you to pace.
For the next 50 of this 100 swim smooth and strong, keeping your breathing level, and not overly exerting yourself too soon.
For the next two 100s (200 to 300 marks) keep the same smooth stroke, but slightly faster, building speed into turns and each successive 100.
For the fourth 100 you should start to really build into your swim, focusing on speed, with an increase in stroke rate, while keeping the same level of kick from the first part of the race.
For the final 100, and especially the last 50, you should be moving as fast as you can. Your kick should be at the fastest it can be, and the last 25 should have as few breaths as you can manage.
Just Plain Old Going Out Fast!
At the risk of becoming too tired to finish the swim strongly, there is also the option of taking out the swim fast from the beginning.
If you have trouble consistently keeping pace, or find yourself unable to build your speed over the duration of your swim, starting out fast may help.
Fast does not mean all out sprint, but instead around 80-90% of your maximum speed.
Maximize power off of your start and turns, using the beginning of the race to try to build a lead.
As the race goes on, and the other swimmers start to pick up speed, try to maintain your lead as best you can
Give everything you have left in the last 50, increasing stroke rate and picking up kick.