this is how i boosted my 3x locks to 80 (using a ret pally 3.9K GS - nothing special)

1-20 Questing (3x locks on their own)
20-25 Shadowfang Keep
25-40 Scarlet Monastery
40-45 Zul Farrak
45-58 Scholomance
58-59 Outlands Questing (3x locks on their own)
59-63 Ramparts
63-68 Sethekk Halls
68-74 Northrend Questing (3x locks on their own)
74-80 mixture of Instances, PVP and Questing