While riding I met Ben who has continued his weight training regime and maintains the muscle mass while cycling 300km per week.

What it interesting is that there is a belief that high levels of cardiovascular fitness training can work against a person wishing to maintain the hard work performed by weight training.
Well Ben is testament that after a long period of weight training, in Bens case 20 years, the muscles develop a memory meaning that after a long period of resistance training, the muscles will not contract or turn to fat. With regualr training the muscles become stronger and more efficient at processing contractions. Your brain creates these neurological pathways, similar to my field of wheat example whereby if you keep walking down the same pathway, these become deep well highlighted tracks that are easy to follow.
Think about a person who has been jogging through out their life and they take a break for 6 months, when they resume jogging the time to regain there formal fitness levels are shorter to someone who runs infrequently over a similar period.
The main thing is to maintain regular weight training and manage the energy intake of carbohydrates, protein and fats.
If the equation is skewed to greater energy expenditure than intake, you body will use the energy stores of glycogen, fats and then breaking down muscle for energy, not a good thing. This process is called catabolism where by the body utilises other form of glucose in the body for energy production and starts to break down muscle tissue in the absence carbohydrate.