Discipline is only a small part of the answer and IMO the wrong place to start with, especially for people who have never had good fitness habits. You can only begrudgingly force yourself to do difficult tasks for so long. That's why fitness gyms see big influxes during the new year and then traffic dies off a month later. It's not necessarily that people aren't disciplined, its that they're bored.

What's more important is finding activities that are naturally motivating and fun to do. These habits are far easier to build when they are more rewarding. Rock climbing, squash, swimming, martial arts. If its fun, then you WANT to go more often. You actually think about improving your skills. And then you naturally build discipline because the cost of missing out becomes more expensive to you.