Well, OK then, but the way you phrased it betrayed a frightening level of ignorance about the differences between 5k and the marathon. If you want to race longer distances, then go ahead and give the 5k a try, and maybe jump into a 10k or two. Work your way into 15k/10 milers and eventually a half marathon. If they all go well, THEN you can think about racing a marathon.
Of course, you can go ahead and try the marathon straight out of the box (I did), but you will fail miserably (I did) if you treat it lightly (I did) and fail to prepare properly (I did).
In all seriousness, what do you want to know, or are you just trolling for a reaction? The simplest starting point is to start working at building up more mileage (whatever you can manage, but more than 30-40 for sure) with tempo/fartlek a couple times a week plus a longer run. Then when you're feeling good, start doing some faster workouts (once a week something at or faster than 5k pace like short repeats, once a week something slower than 5k pace like long repeats or tempo).
Then have a go at racing a bunch of times, and assess where you're at.
Then repeat the process, more or less, trying to get the mileage eventually higher and the workouts gradually faster (but not too fast for current fitness). Always finish a workout feeling tired but knowing (for sure) that you could have done a little more if the coach insisted.