Discipline, and mornings, is the key - as others have said.
The truth is, nobody wants to get up at 5.30am for a run. Not even super-fit marathon running types who have been doing this for years. I'm well aware that's probably what I look like to passers-by when I'm doing my morning runs, but the reality is I'm still having to peel myself out of bed in the same way as when I did c25k all those years ago. I'm just disciplined and know that if I don't get it done in the morning, it'll never happen, as evenings are a chaos of kids/dinner/jobs/kids clubs/etc. I lay out my kit the night before, up and out after a coffee, and am back in time to sort everyone out for the day ahead.
Also, discipline. You just make yourself do it. I've coached c25k courses before and the ones who finish it and go on to be runners aren't the ones who look like "natural" athletes - they're the ones who come out on the rain, or in the heat, or when they're tired, or when they're busy. They keep showing up. The ones who have excuses never finish; and that's because we could all have excuses if we wanted to. People who exercise daily aren't less busy than anyone else - they just prioritise it.
Finally, don't exercise with a friend, or at least not all the time. You're halving your chances of success if you do, because if they cancel then you'll be tempted to cancel too. Make this something you do for you, and you'll be more likely to stick at it.