A Walter Mitty character is an unexceptional person who is prone to daydreaming of personal triumphs.
Someone who goes to town does something enthusiastically and as completely as possible, especially if this involves spending a lot of money.
If you take up the reins, you assume control of something- an organisation, company, country, etc.('Take over the reins' is also used.)
If you make a good fist of something, you do it well.
A charley horse is a stiff leg or a cramp, especially in the leg.
There was a battle in Waterloo, in present-day Belgium on June 18th, 1815, which Napoleon lost. If someone has "met their Waterloo", it means they have been defeated or met their death.
When it is raining cats and dogs, it is raining very heavily.
If something is a distraction from the real issues, it is a red herring.
The rat race is the ruthless, competitive struggle for success in work, etc.
A salty dog is an experienced sailor.