Variety in your fitness routine
One problem with any fitness routine regardless of how hard it is to start with, is that your body adapts — very quickly. So its very important to vary what you do. To a large extent it depends on your goals, but assuming the general idea of why you are training is to make you look better and move better, then variety is essential.
So if all you do is run, then try doing some high intensity interval training (HIIT), or strength training. If you are running long distances, try shorter distances, sprints, and hill climbs. If you are just strength training, put some cardio into your strength training with HIIT. Keep changing it up, or you’ll adapt, overwork certain muscle groups, let other muscle groups atrophy and this can lead to injuries.
Most importantly, keep moving. Sitting is still the worst thing you can be doing.