Will I need to build up my readiness level again to get the good ending on the extended cut?

Solution 1:

In releasing the Extended Cut, EA/Bioware also reduced the EMS requirement for the various "endings". Specifically, the cutoff is now 3100 EMS, down from 4000 (With 50% readiness, this is a sizable 1800 reduction).

Readiness, in this case, is irrelevant. The only thing that effects the ending is EMS, which Readiness is but one part of.

Since you have to resume from before the assault on the cerberus base anyway (the same place the game kicks you out post credits if you choose to continue), you can still fly around and pick up as many War Assets as you have left to claim.

You'll have to judge for yourself if you can hit 3100 EMS or not. (If you assault the Cerberus Base with at least 3050 EMS, you'll clear this threshold)

Alternatively, have you considered simply watching the endings online, on a video service such as youtube? It still saves you from having to replay the rest of the game, and figuring out if you can get the best ending.