Ask HN: Taking a job I feel under qualified for?

Throwaway account. I have been working in a very steady, but unrewarding position over the past 4 years. Initially, I loved the position, but after the first year, I didn't learn anything new, and work became extremely boring and monotonous - most of the co-workers I would consider friends also left for other positions. Recently it became so soul crushing that I decided to start applying to other jobs. I did 3 interviews and got 2 offers. I declined one offer because the job was similar to the one I hate now, but I accepted the other offer because it's exactly what I think I want to do, and the pay and company are amazing. My only fear is that I feel under qualified for the position. My thinking is that I should just go for it, and that I will learn it. I don't really know what they expect me to know, but I just feel that it's more than I do know. I interviewed with half the office over 2 days and they wanted me to start ASAP. Part of me says, you might as well try this because if you're still at your current job in a year you're going to be depressed - and part of me says, you're a fraud for accepting this job, and they will figure you out and fire you - and part of me says, you will fuck up in the beginning but if they work with you, maybe you will stay and do well. I'm really stressed and don't know what to do - I already gave my resignation from my current job today. I just don't feel like I have many fallback options if this doesn't work out.