In order to make anything a habit, it takes about 3 weeks, minimum. It takes doing something consistently for 21 days. Usually, new habits are difficult to form because they often require a person to do something, that is out of their comfort zone.
If it pertains to fitness, I want to express caution with thinking about intensity before thinking about consistency. Consistency is what needs to be priority number 1. A person should realistically set a certain frequency of exercise per week. One must ask themselves, how many days do I have to exercise this week? If I can prioritize (yes, it must be a priority and planned) working out 3 days per week, then I will not try and tell myself I will make 4 days happen.
If I tell myself I will train 4 days a week but can only realistically train 3 days, I will only be setting myself up for failure. Failure to hit realistic goals can be discouraging to many people and could cause a new person to give it up before they really ever started.
Walk, walk, walk. The most functional and underrated exercise that a person can perform is something we do everyday. Try walking after your last meal of the day. A lot of people want to watch television or relax after they eat. I challenge people to walk after meals or their last meal of the day. Do 20 minutes of walking that is specific to exercise and not shopping, or work related, etc. Walk for exercise, 20 minutes per workout-day (ideally walking everyday would be preferred).
If you prefer training at a health club, by all means. Walk into the health club, do 20 minutes of exercise (low intensity) and walk out! Get in, get out. The theme is consistency and developing habit. Intensity can come after 3 weeks of a consistent frequency of exercise has been met.
Good luck & know that health, fitness is a journey with no destination. It is a lifestyle change and should be treated as such.