The start of the (athletic) year is a great time to boost your performance.
Now, at the start of the new season, is a good time to do just that. Your goal race should be more than half a year away, though. If not, do this after your goal race, at the start of your new training cycle. - Why? Improvement needs time, so do specific preparations for your goal race. Pick the weakness that has been bothering you the most and work on it (or the one with the highest chance of improvement).
I can hear people say, I am a runner, all I do is run, so how could this benefit me? Well, even when practising a single discipline there is some aspect that can be boosted. For runners that could be cadence, stride length, an-aerobic power, non-specific (general) endurance, ... You would focus on the aspect in question by adding a few specific workouts to your weekly training regimen.
Said principle works for every sport and that is also regardless of level and age. If in doubt, ask your coach. If self-coached, you know already what I mean.
I guarantee you, if done properly (and repeatedly over the seasons), such an approach will make you a more successful and balanced athlete. And that's just what we want, happy and successful athletes.