I started running as a side line to cycling at the end of 2011 and ran my first marathon in 2013. I still run, as a side line to cycling. If that's your plan, then read on, if not, pick up Runners World.
There are three things that you need to run a marathon.
1) Aerobic fitness - as a cyclist you will be aerobically fitter than a runner, especially if you ride up a fair number of hills.
2) Endurance - ditto - because cycling doesn't bash your body about 100 times a minute as your feet strike the ground - you can ride for several hours and still walk afterwards - something that is very difficult to do if you run for such a length of time.
3) Have a good running technique.....
That's the hard one. Now for my dislike of the advice that tends to be given by running magazines. They assume that you are going to achieve (1), (2), and (3) by running. But if you cycle 2 -3 times a week and you run four times a week you will have no chance for rest. Also, if you run four times a week you will have a maximum of 48 hours to rest those poor legs (and the rest of your body) from bashing the ground 100 times a minute. Also, on the shorter runs that they suggest you have more chance of getting away with a poor running technique and developing habits that will injure yourself.
For the running technique start off by getting the rights shoes. I go to a serious running shop (Alton Sports) where they video you running and make sure that you have shoes that support you in the right way when you are running. Second - follow the advice repeated many times above
Do the long runs.
In fact, that is all I do. ( I have a long term injury where the tendons at the top of the inner leg attach to the pelvis (one of those things that generally go under the term a 'groin strain'. Running makes this injury complain, but after four or five days its fine. In fact running once a week seems to strengthen that area. But that stops me from following the runners weekly approach of 4 runs a week).
Its on the long runs where you will get into a good balanced run. You will also test those muscles which cycling doesn't reach, typically the ones that give sideways stability. You will also find that that 100 times a minute bashing jars up the body and the long runs build the endurance that help you deal with that. If you are just starting out that long run might just be 2 miles (that's where I started in November 2011), but then builds progressively from there. I'm a few weeks into building up to my next running challenge, which is more than I thought it was going to be as they've upped the distance this year from 33 to 45 miles). So this morning was a 14 mile run (after 7, 9, and 12 in the previous 3 weeks). In my previous training pattern I would have dropped back to 12 miles next week and then progressed with 14, 17, and 19, to gradually build up my body's tolerance to running and deal with point 3 above, but this time, knowing my running legs a bit better, will progress to 17 next weekend.
The final bit is stretching. I'm not a big one for stretching after a bike ride but am very discipline after running - because of the 100 bashes a minute factor the legs can get quite badly jarred and the muscles tense up accordingly. So this morning after nearly two hours running I spent 25 minutes stretching. Mostly hamstring and quad stretches, a few calf stretches, some specialised stretches taught to me by a physio for my dodgy groin and then a foam roller to ease out the tension in the IT band (that bit that goes down the side of the leg and isn't used much in cycling but is critical for running).
But running will still be a sideline to cycling - the big goal is Paris-Brest-Paris in 2015.