Most fitness plans fail not because they are too easy, but because they are too ambitious to maintain. A good routine is realistic, specific and flexible enough to survive a busy week.
Pick a single focus to start: building endurance, getting stronger or simply moving more each week. A clear goal makes it easier to choose activities and notice progress.
A balanced week might combine two strength sessions, one or two runs or rides, and one easier day of swimming, yoga or walking. Group classes can add structure and company if you find it hard to train alone.
Increase either the intensity or the amount of training, not both at once. If you are preparing for a race, build weekly distance step by step and keep easier recovery days in the plan.
Learn proper technique, and stop if unusual pain appears. A missed session is far less costly than an injury.