Fastest way to get them, and really the only way, is to do your daily and weekly activities. You get 30 a day by doing heroic missions and the daily crucible playlist, and 50 or 60 a week by completing the weekly crucible playlist and your heroic strikes. You get 25 marks per weekly iron banner bounty if you turn them in, 25 legendary marks for first time completions in several of the quests in the TTK quest line, and at least 3 marks for dismantling a legendary item. I may have missed something, but these should be all of the possible ways to get legendary marks. Just by doing the daily crucible and heroic missions, you can get 210 marks a week.