First, know that many people get promoted in this sort of way - two of my former managers were. Whether this is good or not depends on the person - but you can read about management and project management and get the company to help with leadership courses and the like to fill in your knowledge gaps. To do this you have to take a deep breath and believe in your ability to overcome.
That being said, management is not a good fit for many people, and having it thrust on you in this way is very stressful - if you think you are truly not up to the challenge (and kudos to you for recognizing it...it can be difficult), you need to talk with your managers. The best thing you can do is convince them to hire for the management role. The best specific approach for this depends a lot on your communication with your management and your company culture, which I do not know and cannot get into here.
If you do not wish to get into management, there are two major possibilities,
- A. you do not want to get into management ever
- B. you simply feel you are not ready for that challenge.
If A, you would stress this to your management. You would explain that you prefer being "in the code" and do not really want to be a manager and think it would be best to hire in a fully qualified manager to take that responsibility.
If B, stress your relative lack of experience managing and say that you think it would be best to hire in someone who has more experience managing (as Guy Schalnat noted you could request a program manager specifically). Since you mentioned they are looking to hire more technical staff, you should be able to persuade them to hire a manager in as one of those people.
