It's funny though because you only need a short amount of discipline. Because after you use discipline for a while, it just gets way easier as it becomes a habit. Reminds me of this great Bojack Horseman clip - https://www.youtube.com/watch?v=R2_Mn-qRKjA
Years and years ago I started training seriously for a triathlon. Then a half ironman. Then a marathon, and after 2-3 years of having a few races a year on the calendar, I just started exercising every day. At the start, a lot of the days took discipline/motivation to get out the door. After a while, it was just a few days of discipline a week. Then a few days a month, then a few days a year. Now it's automatic, and something I actually look forward to rather than dread (obviously in a hard training cycle there are some sessions I dread, but it's almost like my brain turns off and my body subconsciously gets my stuff and gets me out the door).
For endurance sports I find it helps to put yourself on some kind of structured training plan leading up to each event. There are lots of free or cheap training plans available and any of them will work well enough for a casual age group athlete. This removes the daily decision making process and allows you to focus on execution.
Years and years ago I started training seriously for a triathlon. Then a half ironman. Then a marathon, and after 2-3 years of having a few races a year on the calendar, I just started exercising every day. At the start, a lot of the days took discipline/motivation to get out the door. After a while, it was just a few days of discipline a week. Then a few days a month, then a few days a year. Now it's automatic, and something I actually look forward to rather than dread (obviously in a hard training cycle there are some sessions I dread, but it's almost like my brain turns off and my body subconsciously gets my stuff and gets me out the door).
The longer you do something the easier it is.