This is the time of year that people resolve to get in shape, then decide that they'll do that by running a marathon, then choose a race 6-10 months away that they'll turn into a reward by making it their year's vacation destination and then look up a training plan to get them to their goal. I get it - it's just not what this blog is all about.
The most common mistake I see beginning runners make is emulating top runners when they're not ready and that includes spending months training for one race. Until one is a veteran racer - a minimum of three years and 30 races - one should go about races differently. Race when you feel like it and race often; you'll improve quickly just from the experience. Most importantly, race as great a variety of races as possible - road, track and trail - at as many distances as you can. I'm lucky enough to live in an area that has a race every single weekend and often dozens from which to choose in the summer and it's still possible to enter some races the morning of the race (race directors hate this, but after 2 or 3 years of holding a race, they should have a feel for how many entrants they'll get and how many will be race day and it's not a big problem). Getting in the mindset of "I want to race today!" is far better than the "I'm not sure I'm ready... and the weather sucks" mindset that often comes from entering a race too early.
After you've done a large number of races, you'll probably discover that the race distance you enjoy the most, the one you do most often and the one you're best at happen to all be the same. That doesn't always happen; my best distance is 3K, a distance almost never held, and it doesn't mean that that's the only distance you should run - for example, I'm currently training for a race 15 times as long as my best distance - but it's a guideline for the distance you should focus on doing if you want to race well.
When racing, keep an eye on your position in the last third of each race. If you're passing people, you might be better suited to a longer distance; if you're getting passed, you might be better at shorter distances. If you're passing people in every race, regardless of length, you're starting too conservatively (and, if you're getting passed in every race, you're starting too fast - I get accused of this, but in an 800m race, I'm trying to make up ground in the last half).
One way to make a guess as to what distance you'd be best at is to consider your build. Sprinters are tall and muscular, middle distance runners are tall and thin, marathoners are short and thin, ultramarathoners are short and muscular; the exceptions are so many as to make this at best a very rough guideline. It's pretty basic - sumo wrestlers, jockeys and high jumpers would be terrible at each others' sports.
If you want to get technical and find your best distance, the simplest way is to divide your times by the world records at each distance. The closer you get to 1, the better you did. You can skip the math and make corrections for age and gender by using online age-graded calculators. Here's a link to a good one: [calculator] Just plug in your race times. You should find that you do poorly at very short distances (unless you're a sprinter), then get better until you hit a peak and then get worse and worse (unless you're a long distance specialist). If you haven't done many races, your numbers will probably be all over the place and you won't be able to see what you're best at. [If you plug in my personal bests from the very bottom of this page, you'll see I should stick to races under 10K] That in itself should tell you something: you're still a beginner, despite how many races you've done and how long you've been running.
The ultimate way to find one's best event is laboratory testing, but that's a two-edged sword, as you may find it deterministic: "I'm a sprinter and there's nothing I can do about it. I stink at the distances I want to run and there's no hope." Muscle biopsy can determine the percentage of slow and fast twitch fibers and that is perhaps the best method for finding one's ultimate distance - but it is a surgical procedure; costly, painful and harder to recover from than anyone says - and it is still only one measurement among dozens that can have an influence. One thing that's been found is that the percentage of slow twitch fibers in a population follows a bell curve; half of people will have 50% or more, only one fourth will have 75%, etc. and thus only 1 in 20 runners has any business trying to race a marathon and maybe 1 in 50 should be racing ultras. [I'm 43% slow twitch, which is about right for a 3K runner] Given the millions of people who run, that's still a large number of people best suited to racing marathon distances or longer. It's just not everybody. And, if you're not ideally suited to doing the distance you want to do, you have a lot of company; you can still do the race, just know that you probably won't do as well as you would at another distance.