Mind I personally never trained for anything...
However having been in relationships with guys who took the whole training thing to various levels of seriousness, and having been a gym mouse (I went along to offer moral support and to be a 'spotter') I did pick up that the ends determines what it is one does in the gym.
Mass building exercises and 'toning' exercises use two different mass of weight in order to work the two primary different forms of muscle.
The two broad types of skeletal muscle are slow twitch and fast twitch.
Fast twitch gives ones explosive strength, say the ability to heft a lot of mass once. This form of muscle tends to grow a bit bulkier than slow twitch muscle. Thus Mass training is high weight and low reps to work those fast twitch muscles.
Slow Twitch muscles tend to be 'weaker' in that they do not lift as much mass, but they have endurance, able to lift a little weight over and over again. They also are not as thick as the fast twitch muscles, which leads to being able to cut and define visible muscle.
You need fast twitch muscles to throw something a distance. or to lift a heavy mass once, or twice.
You need slow twitch muscles to run marathons, to do repetitive muscle movement over and over again.
Most body-builders who take the whole subject seriously, break up their year into 'seasons'. One season they lift heavy weight, low reps and sets (mass building) the next season they lift lighter weight, higher reps higher sets for cut and definition and a bit more endurance.
In the case of marathon runners, the 'ideal' is to have a compact low weight body, your muscles are worked far more on the slow twitch side of things, thus you will not heap on extra pounds. This allows you to run further and longer.
The 'seasons' method also does other things, such as it prevents plateauing (stagnation), lends it self to improved core strength, gives the person real strength, as compared to 'weak muscles' which are all show. It also improves the cardiovascular system.
This method also helps in burning intramuscular fat. As you may know there are three basic forms of fat, subdural (the fat right under the skin), intramuscular (look at a steak, that white and red marbled appearance is fat inside of muscle) and organ fat. Low weight, high reps/sets tends to burn more of the fat as it requires more energy over a longer period of time to reach failure of the muscle (feel the burn type failure).
You may want to seriously consider taking your program more toward defining and endurance training for now. This will work to your advantage later down the road, and have immediate benefits, such as allowing you to have better control of free weights which leads to reduction in sports injuries.
I also suggest more cardio as you are going to need a strong heart and lung capacity to run a marathon.
I doubt you will lose much mass in the course of a year. you most likely will see a shift of where your mass is, as in more muscle mass and less fat mass as you build muscle and burn fat. I doubt you will end up 'skinny' doing a low weight/high rep/set routine.
Mind every marathon program does require you run/jog and build up the right leg muscles to run effectively and efficiently for long periods of time. Again its the fast twitch Vs Slow twitch muscle.