Maintaining your fitness is harder than gaining it.
Anyone, and I mean ANYONE can take 12-16 weeks and dial in the training and nutrition and look fantastic. The beginning stages is the easiest to see change and progress This is not the issue.
The issue is maintaining your fitness that you have built up. The saying is "If you don't use it you lose it." This is so true. People get way too comfy once they are in shape that they forget what it took them to get there. The body will only hold physical fitness if there is a stimulus on the body telling it to. To make a long story short, you have to continue to exercise and eat right in order to stay in shape.
It is that simple. You either hold onto it or you do not. Often times mentally after a period of time off it is harder to get your strength and your health back. Consistency is crucial no matter what stage of the game you are in. Not only will you maintain your health but you will also make gains and progress and advance your fitness further!