You will need to have the level four training skill to use timed workouts. Timed workouts are a very useful tool when training your horses. When done properly, they can improve Courage, Consistency, Morale, Prime, and even Risk. With unmaxed horses, they can also be effective at training stats. However, they are more of a gamble than other types of training because poor quality workouts, where the buddy scores significantly higher or lower than the horse being trained, can have negative impacts on your horse. The good news is that a bad workout or two isn't the end of the world - keep doing workouts and the good ones will repair any damage done from the bad ones!
Doing a timed workout is simple. All you need to get started is a buddy. Buddies are retired (not pensioned) geldings. If you're working a young horse for the first time and know nothing about how they'll perform, select your lowest statted buddy for the job. A horse with low Peak may perform very poorly in workouts, and a lower statted buddy has better odds of being a match.
Select a distance and surface suitable to your horse's current training. You can select any style, as this will determine which stats (if unmaxed) will improve. A workout history will be kept for every workout your horse performs, allowing you to see improvements over time. If you notice that they didn't run well with a buddy, adjust next time. A horse scoring lower than their buddy needs a lower statted buddy (or more time to mature before they do workouts at all), while a horse scoring higher than their buddy could benefit from a higher statted buddy.
A horse can only do one buddy workout per month. It will not take energy away from the horse who is buddy worked but will take energy out of the buddy so remember to have a handful of buddies of every potential range so you can workout all your horses.
As you can see on this example, the first buddy was picked based on the horse's potential but gave a negative result as the horse in question was running scores much worse than his potential. Therefore, another buddy of a lower potential had to be used to have a positive effect, and it took a few tries to get there! You should aim to have your Horse and Buddy's score within 10 points of each other, and adjust up or down the Buddy potential depending on your results - for example, if your horse begins to outperform your buddy, it's time to move up to a higher potential buddy.
Buddy workout are important, especially for G1 graded horses because they will give experience. Experience is a visible marker of campaign strength. With horses who aren't competing in G1 races you don't need to pay any attention to it at all.
Racing will always be the largest contributor to experience. Horses will earn experience the fastest by entering G1 races and running high PRs. All buddy workouts will provide smaller experience gains, but they'll gain more when they run high workout scores.
You should always buddy workout horses that will run in major G1 races (gems, trinities, GC, WCF) to gain experience so they will perform better.
Scores are really simple. The higher the score, the better. A high score essentially means your horse ran their workout with a faster time. The unofficial high score is 100 and the score system could be viewed as a percentage of perfect. Ex: A 50 score is 50% of the maximum possible score. However, it is possible for horses to work scores over 100 when they're running sharply - these are horses who are really at the top of their game!
Generally speaking, you'd like to see G1 runners with scores of at least 85 (90+ is better). G2 horses won't run too far below, while G3 horses may dip into the low 70's or even upper 60's on occasion. In most cases, horses running under 50 are either not close to ready to race (Peak wise) or have really, really low stats. One high score does not necessarily mean a horse is suitable for G1 races, so you should still refer to peer comparisions and actual PRs when deciding what grade to enter your horses.
On the flip side, low scores are an indicator of grades for racing. If your horse is a 50+ potential running well below 85 in workouts, it is definitely not ready to run in G1 races. Use the workout score to get an idea of how they're going to perform before you race them, and then drop them into their first race accordingly. Assuming you've already maxed out Morale, low scores are almost always indicative of low Peak. Occasional workouts (even with "bad" results) can be useful in keeping tabs on a horse's Peak. As workout scores rise, you can get an idea of if and when your horse might be nearing Peak.