Training myth or training fact?
If you stop training, your muscle turns to fat.
Physiologically, muscle can do one of two changes - it can atrophy, or get smaller, or it can hypertrophy, or get bigger. It cannot become antipose (fat) tissue.
The reverse is true as well - fat cannot become muscle. It can be used for fuel by your body, but it can't become muscle.
So physiologically this is impossible. Your muscles never become fat. Never fear that if you train now, it'll become fat later. All that does is help discourage you from training, for fear that any lapse will make your hard work undo itself.
So where does this myth come from? It probably comes from a simple cause-and-effect situation. If you strength train effective and eat appropriately (in other words, eat a lot!), you can strength and muscular size. If you stop training, but continue to eat as if you hadn't, your muscles atrophy and the extra nutrients are stored as fat. So your muscles shrink, but you add more fat. This may appear to be "muscle turning into fat." But it's not. It's just a failure to match diet to activity level. If you stopped training and ate less, you'd still suffer muscle atrophy, but you wouldn't gain fat.
So get out and train, and don't worry about it later transforming itself into what you want the lease - excess body fat. It can't, and it won't.