It's not that difficult to level up quickly if you know how.
1) If you know how (and are slightly lucky) you can do two DSE missions in under 5 minutes. That means in under 5 minutes the Borg Deep Space Encounter missions will net you an easy 1000 xp.
2) With the exception of some of the normal misisons, there's little point in playing them on a second run through. They reward little xp for a lot of time and effort.
3) On the otherhand, some of the series episodes reward you with good xp (that levels) for something that takes very little effort or time. For example Spin the Wheel. Most of them also give decent rare equipment too, which you can either use to equip yourself or sell on the exchange.
4) PvP is a great place for some fun xp and cash (as well as medals to spend on uncommon equipment). The biggest problem is the queues at lower levels are only really active at certain times.