Whereas in vanilla Destiny buying gear with marks was your main way of leveling up, legendary marks in TTK are more of a secondary gearing-up mechanic, allowing you to purchase a piece of gear you may be missing. Instead, the marks are more useful for infusing gear that you find by actually playing the game, with strikes being your best bet (I haven't seen the Court of Oryx, yet, so that might be a good option too).
Basically, don't think of it as you having to grind all these marks to get better gear. Instead just play the game normally, doing strikes, quests and whatnot, and better gear will naturally come to you.
I'm only 244 light right now due to not being able to play that much yesterday (adult working life yo), but this seems to be the best advice based on what people have said in this thread and what I've seen in streams.
Edit: Also obviously once everyone starts getting to higher light levels they'll start caring about the perks on the gear and what setup works the best, which may require spending more marks on legendaries and exotics to get exactly what you want. But when everyone is just trying to gear up, spending marks on gear seems to be a waste. Save them for infusing and getting that perfect legendary/exotic later.