Even missions dont get u to 20, bungie expects that if you do missions alone you will get to around level 16, which is why thats the level of the strangers rifle you get from doing it. I did missions and bounties up to 2nd mission ob mars, by then was about level 15, did pvp and bounties till 18, then strikes and bounties for 20. Fastest time was 9 hours, with about 30mins afk and 40 minutes in loading screens (plus 5 minute reef cutscene)