The problem is that as Jehovah's Witnesses we are taught to obey and revere the WTBTS as appointed by God and to an extent the elders. Just because you realize that this is not the truth, that conditioning does not automatically go away, that takes time. You actually owe these people no explanation, but it's probably hard for you to see that now. And when you confided in that elder, you assumed that because he knows you, he would realize you had no evil intent. Unfortunately, his conditioning prevented him from seeing you as just a friend with questions. He is now seeing you as someone who is questioning the status quo, and that set off his fear of Apostacy.
If you want to control the timing of you exit, you will have to do damage control. You do not have to lie, but you will have to control what truths you tell. You could call back the elder you confided in and tell him that you afraid he got the wrong impression from your discussion, and that you did not intend for him to think you are apostate. From your comments, that is true. If he asks for details just say you are still thinking about things (also true), but you appreciate his understanding. Just keep in mind that this religion sucked you in with lies and cover-ups and you are entitled to get out of it with the least impact to yourself.
Practice a few stock phrases in the mirror until saying them becomes automatic, so you are not caught by surprise and blurt out things they are not entitled to know. These may be nice people, but they are conditioned to fear Apostacy, so you have to be very discrete.