I thought our defense went from 'good' to 'great' as the game wore on. I don't know if it was the early rust or just the unfamiliarity with the Heat that took a while to shake off, but the Spurs put together a 3rd and 4th quarter where they only allowed 36pts combined for that half. While the Heat is sure to look at the tape and adjust the offense to counter some of the Spurs' schemes, I also think the Spurs will watch the tape and see where they can improve over what they did.
It's difficult to keep that defensive intensity throughout, but if the Spurs can put 3 solid quarters like that, they're going to give themselves a great shot at winning game 2 (or any other game this series, tbh).