Key back into the ignition and turned it to no avail It would not crank over?

I started the car using the auto start to warm it up a bit. Entered car and put key in ignition. Drove approx. 6 miles to the mall and parked. My wife turned the car off, took the key out and exited the car. I was staying in the car so I called to her for the key. I put the key back into the ignition and turned it to no avail. It would not start. I had lights, radio etc, but it would not turn over. I got a very faint crank and that was all. I wound up getting a jump and l between leaving it run and driving to a few other places, it ran non-stop for at least and hour and a half. The problem has not reappeared since. Between the time my wife took the key out and I replaced it to try and start it again approx. 2 minutes had passed. The battery is about 2 years old. Any suggestions as to what might have caused this.