Right, I often play blitz games for an hour a day weeks on end and don't improve at all. Interestingly, looking at professional games, even if I don't bother to calculate many lines, seems to make me slightly better; so there are ways to improve without deliberate practice, but playing blitz doesn't happen to be one of them. Playing standard time controls does work decently well though, at least once you can recognize all the dozen or so main tactics.