BTW daily pvp and story are account wide now, so you only have to run each once per day, no need to run it on each character - won't get you anything extra. Thats 40 legendary marks/day if you do it every day, plus whatever you get for nightfall and weekly heroic. Its good imo, it would be kinda insane to expect people to run each 3x for 120 marks a day, taking up 2 hours.
The story mode will take about 6-8 hours the first time you run it through and probably 4 hours or less for the second and third times. A lot of grind-y type stuff opens up afterwards, like there are 50 ghost shards on the dreadnaught you have to find to build an exotic weapon, or a bunch of long chain quests involving patrols, story missions, old raids, etc.
My titan ended up finishing the story at light 224, thats with a lucky drop from two faction packages (weapons) and shoulders. I guess if I get 40 marks today, tomorrow, and wed I can buy a piece of armor and maybe something from xur on friday and hit... around 260? Not sure how someone is going to get to 280 light without a lot more grinding and/or getting super lucky with faction packages or engrams.