A fit person walking briskly can walk a mile in 13 minutes. An estimated average walking speed for calculating... An average person can walk one mile on a flat, well-groomed surface in 17 to 20 minutes. A fit person walking briskly can walk a mile in 13 minutes. An estimated average walking speed for calculating...
Mum's the word is a popular English idiom. It is related to an expression used by William Shakespeare, in Henry VI, Part 2. Mum's the word means to keep silent or quiet. Mum is a Middle English word meaning silent. The word may be derived from the mummer who does pantomime and just acts without saying anything. Note the phonetically similar German word stumm (Old High German stum, Latin mutus) meaning silent, mute.
The second use of 'Mum' is capitalized because here you are using the word as her proper name. Capitalization is not related to whether or not a word is abbreviated. You'd also say 'I'm happy for Mother'. The word 'mum' is not a pronoun. Best wishes, Clive.