Wait wait... if all you are looking for is MARKS and NOT REPUTATION, then the fastest way for you to get them is to go to Cosmodrome (Earth) Patrol, go to destinypublicevents dot com and farm public events. I believe you get like 3 or 4 marks per public event that you do and on earth they are pretty frequent. That website will help you find the events (they developed an algorithm - trying to map a pattern where the public events happen) read the instructions and go do the public events. If you do it right, you should be CAPPED for the week in just 2-4 hours. Plus, you receive a package with some goodies for the first public event of the day that you do, AND you have a chance to receive ascendant materials (shards and/or energy) for each event. Don't get off your sparrow when traveling between events either if you can help it to get to them faster, because some are really close to each other. And remember - READ the instructions on the public events website so you know how to properly search for them (greens are common, go to those, don't bother with reds, and only bother with orange if no greens are close by at the time - and MAKE SURE you wait until the -3 minute mark, I've found that's when they usually spawn).

Just keep doing strikes... Best way to get to 28 is tiger strikes night and day... Playing crucible help getting from 0-20. Also bounties are the cornerstone to this game. Make sure you plan out how you will do them in order to get them done as efficient as possible. Destiny takes a lot of your time... Time a lot of folks don't have...

Thank you brother! I've been knocking out crucible thinking it was the fastest way.

Yeah you get 2000xp for every game even if you lose... A game is about 10min... You can get 3 games in before you even get most story missions done. Then you can go through the story at level 20 all on hard and won't come close to dying.