The wife and I are holidaying near Ambleside later this month and we'll be taking our Ventana tandem. I'm looking for a couple of routes that would be doable, I'm very confident but she isn't!
Preferably all off road and around 15-20 miles. I'd prefer not to just do Whinlatter or Grizedale, but have listed them as backups.
The skiddaw loop would seem to be a good choice from keswick.
From ambleside you could try the trail over Jenkin Crag into Troutbeck, then over Garburn pass and back down the west side of the Kentmere valley
Or there's the usual loop over to loughrigg, past the drunken duck, iron keld, through hodge close quarry to the end of Langdale and down to Rydal water along loughrigg terrace
Skiddaw
http://www.bogtrotters.org/route/showrouteframe2.php?route=356
jenkin crag -> troubeck and back (add on garburn if you wish - the singletrack in Kentmere is brilliant though so highly worth it
http://www.bogtrotters.org/route/showrouteframe2.php?route=367
Ambleside map
and MMO file
http://www.bogtrotters.org/rides/2007/13may/ambleside_wander.mmo
nbt has some excellent suggestions.
Our ventana has done the Iron Keld descent (south to north direction) and its reet good. The brideways in the Kentmere/Stavely area are also very tandemable. Only solo'd the full Skiddaw loop but we tandemed the Keswick/Threlkeld/Blencathra centre/Lonscale/Latrigg loop, which is good, but a bit nervy on the Lonscale rocks!
Whinlatter blue is actually good fun on a tandem, and one of the guys at Cyclewise is a tandemer too so good for an mtbtandem gossip, which is something we rarely get!
Enjoy
Rob