If only it were that simple.
'Less' would normally used in front of counting nouns denoting distance, amounts, or time.
Few people would say something is due to happen in 'fewer than six weeks', it would be 'less than six weeks'.
Or: 'the bomb is due to explode in fewer than ten minutes' doesn't work, it would be '...in less than ten minutes'.
'My bike ride today was fewer than 60 miles', no, accepted usage is '...was less than 60 miles'.
There are further complications, some of which I don't claim to fully understand.
Not really that complicated. Some expressions of quantity are taken as singular rather than plural items, for reasons of common sense. Six weeks, ten minutes, 60 miles are being treated approximately in the examples you quote. I appreciate it is a fine distinction, but to pick one example apart, these two sentences make perfect sense to me:
I was tired today; managed less than 60 miles. (Approximate statement of a satisfactory distance.)
Riders who cover fewer than 60 miles will not qualify. (Precise number needed for specific purpose.)
When we say 'ten minutes', we are usually referring to a vague chunk of time, rather than 600 seconds precisely.
The rule I gave is easy to understand and covers 99% of situations. The examples you quote here are what makes the study of language so fascinating.