Success is sweaty. And it takes a plan. A training plan.
I can't live without mine. Coach Shane chose the plan I'm currently using as I prepare for World Champs and it makes life simple. I know I'm doing the right sessions and right recovery to get me to my goal; my only job is doing the work well.
My first plan was a paper one; how gratifying it was to cross off each session as I completed it. Then Coach moved me to Training Peaks (www.trainingpeaks.com) he can monitor my progress and tweak my training. Both Training Peaks and Garmin offer plans that work with the Garmin watch I have; but Training Peaks currently has a larger library of plans.
If you've never used a plan, I can't recommend one enough. When you search for a plan, consider the event you're training for, the level that you're at, how much time realistically you have per week to train and check if the plan's philosophy matches yours. For example, some offer higher intensity with lesser overall miles while others build upon longer distances. Some plans are designed for athletes over 40 (My Pro Coach just introduced one that looks good); some plans focus on heart rate only while others integrate power and pace.
When I first started doing triathlons, I printed out and used free plans. Let's face it: triathlon is an expensive sport - and I wasn't sure what I needed. There are free plans at https://www.triradar.com/training-plans/ . I have even checked out books at the library that contain training plans. But as I've progressed, I'm now willing to buy a plan because I know I'm going to spend a lot of my time and energy following it. I also find it handy to have my plan on my phone, via the Training Peaks app, so I have access to it anywhere. I've used plans by Matt Fitzgerald (author of "80/20 Running") and have been using plans by Phil Mosley (My Pro Coach) for the past year and a half. Because frankly, if I'm going to try this hard and sweat this much, I want to see results.