Originally Posted by burnthesheep

A long sprint and a 15 minute long effort are pretty much diametrically opposed to each other unless you're Wout van Aert. Meaning, you're going to have to pick one. You can improve at both and be mediocre at both, or you can be "competent" at one and really good at the other.

For long sprints or time trial follow the other poster's advice seeking training plans. But just pick one of the two.