In my 30 (+) years of personal training experience, one of the most frequent complaints I hear from soon to be or existing clients is that they are not happy with the shape they are in. I listen as individuals talk about their frustration at the slow progress they are making in getting in shape. They swear they know what to do and are exercising and eating the right amounts.
Usually, with a bit of further investigation, we can both come to the conclusion there is a bit of inconsistency with their fitness programs. Life sometimes gets in the way and workouts are missed and eating gets off track. If they could be more consistent on a daily basis they could see better results. Alas, no one always feels motivated to exercise and eat healthy. To be consistent requires making small goals. Instead of planning to do an hour of cardio every day, try committing to doing 30 minutes. Give yourself that much of a range of possible time to do your running, biking, etc. If you do the 60 minutes, then you have made a great accomplishment. If you do the 30 minutes, you are still doing terrific and have remained consistent.
If consistency is one of the major missing keys to a successful program, as I feel it is, then accountability is a very close second. It can be extremely hard to remain consistent without that accountability. It could be a trainer or a friend who helps you be accountable. Small group exercise classes are terrific for accountability. Not only do you have an instructor to nudge you get to class but there are other members who can motivate you as well. Larger group classes can work but sometimes people get lost in the crowd and therefore lose accountability.
A safe fitness program is definitely built on sound knowledge of exercise and nutrition, which can be successful for many people. The reality is that even with all the knowledge available, most people will not have success unless they have consistency and accountability.