In reply to plyometrics:
> you can extend by going over Garburn into Troutbeck, or Stile Lane into Longsleddale, but that does involve some technical riding, Garburn in particular.
Thinking about it, Garburn east to west might fit the bill if he doesn't mind a bit of pushing. The descent to the west is quite easy.
But defo don't go west to east, otherwise you'll be pushing both up and down!
The fatties site has some good stuff. Doing this route from langdale would probably make it pretty much spot on 20 miles...
http://fattyres.co.uk/routes/lake-district/ambleside-little-langdale/
It includes iron Keld and loughrigg terrace.
> We tend to like fire roads and bridleways.
Bear in mind that "bridleways" includes a huge spectrum of difficulty, especially in the lake district. For example, all the pictures on
https://gbmtb.com/en/lake-district-guided-mtb-rides/
are bridleways, but probably gnarlier than you are after?
Post edited at 20:22