Given that it took many minutes to generate the DAG, I'm wondering if your machine is a bit underpowered. You can make mining much easier by adjusting the difficulty parameter here (lower means faster mining): https://github.com/vincentchu/eth-private-net/blob/master/ge...

It was tuned to yield a block every 15 seconds or so on my laptop; may be slower or faster depending on how fast your machine is relative to mine.