How long would it take to sneak walk 100k blocks in minecraft?

You sneak at 1.31 metres per second. 1 metre is one block. 100,000 / 1.31 = 76,336 seconds. 76,336 / 60 = 12,939 minutes. So on and so forth.

Taking Don's numbers a bit further… Roughly 9 real world days if you didn't stop to sleep, eat, etc.