These might be shorter than 50, but you can add whatever extra bits you like.
Down to coniston - Torver - Broughton in furness - back on yourself to Ulpha then either up to cockley beck and over wrynose... or hang a left and go over the fell to Eskdale and do Hardknott and Wrynose.
Alternatively down the east side of Coniston lake and back up through Grizedale forest to Hawkshead. It's quite nice (and cheap) to cross windermere on the ferry, but that puts you on the busier lakes A roads for a bit. You could head up to Ambleside and tackle the struggle or maybe better, head up Kirkstone, down the struggle on to Grasmere and do Red bank before heading back to the Langdales.
The problem with a lot of the FW route is that its on busy roads and hard to string the good bits together without doing the whole lot!
The good thing about cycling in Cumbria is that its very quiet on the roads, if you pick the right time and roads!
Hope some of that helps.