There comes a point in any project or activity where you're invested in seeing it completed.
If you're interrupted early on you might not make it back. Interrupted later in the process, and the project will live on in your mind with a flashing 'incomplete' light.
You'll have to complete it to get it off your mind.
The 'Zeigarnik effect' describes how it's easier to remember things that we've started but haven't finished, than things we haven't really started or things that have been tied up.
You can leverage this. Want to exercise? Work through the process until you figure out where the resistance stops. Maybe it's getting your running shoes on. Don't think about running, just commit to putting your running shoes on. The rest will follow automatically.
You get it to the point where it's easier to finish it and get it off your mind than it is to leave it pending.
Electronic games often use this to keep you coming back. One more turn until I can... It keeps you playing just that little bit longer, because you don't want to leave things incomplete.
Avoid interruptions if you can. Resuming what you're doing takes time. Instead take advantage of your brain's need to complete things, and use that lever to help achieve your goals.
Once you commit mentally to the task, you'll finish it. Or it will bug you until you do.
Shave and a haircut...