Glad all was OK. I too use Lantus and Novarapid.
If I had woken up to that reading of 12 I would have used my Novarapid first rather than the Lantus which I would have waited to take at my normal morning time. The dosage would have depended on what I had eaten and especially drank the night before but could have been 25 units.
Was it time for your Lantus injection and is 25 your normal dose?
What have you been advised to do for correction doses as I have been never been told to use Lantus?
As others have said get all your back up glucose ready in different parts of the house, car and work. Going out to get some whilst that low would have slowed down your recovery rate even more due to the exercise involved.