I am very clumsy and even used "Restart from Checkpoint" a couple of times to avoid death. This was the 4th time I've completed the game from start to end and my third attempt to do the Hard to the Core achievement, the previous two attempts I've unfortunately died a couple of times and finished both with a time around 1 hour (1:00:31 and 0:59:52 respectively). Naturally, I've practiced chapters a couple of times each after finishing the game for the first time and before starting any full Speedruns.