First, congratulations on your growth! The next step is how to manage this growth.
Take a step back and see what the community is doing in areas where you are not able to keep up any longer. Are there complaints or is the community getting along just fine? Is it "administration" tasks that are not getting done (eg. handling flags, dealing with spammers, updating software), or is it simply conversations you are missing out on?
A solution to both is to add additional moderators. Share a portion of your daily 'behind the scenes' work with others. Depending on your community culture these people can either be appointed by you or voted upon by the community at large. In either case though, with a reduction in some administrative tasks you will have time to step back into the discussion. You should be aware that new moderators may require a bit of training or guidance. This means that your free time won't drop instantly.
If you are missing out on conversations, there is nothing that says you can't provide input a few days later. Much like I am doing with this question, I did not answer it the day it was posted. Yet, you still are getting my input. That is the beauty of the internet. The conversation is there for you when you have time to get to it. However, I'd discourage bumping really old topics.
You can still remain engaged by looking at popular topics and participating in those conversations. Alternatively, to provide a personalized touch though, you could look at the unpopular topics. Who is asking questions but not getting a response? Reach out in those topics and provide an answer. You could set up an RSS feed and filter for less common topics to engage those users more quickly. You don't mention what your community focuses on, but I assume that you have a set of topics that are discussed more often. Filter those out of this feed. Your community will answer those questions, while you can be more personal to the less visited topics.