Ha, I tried this route a year or two ago and said never again! I think you’re supposed do it clockwise? We started at Stonethwaite, it was a clamber up Greenup and the top part of the descent to Grasmere was rubbish.
It was winter though and we thought we wouldn’t complete the route before dark (it was taking way longer than expected), so hit the tarmac back to the start via Keswick.
Your planned ride is definitely a challenge but not worth the effort if you ask me (although the Langdale, Black Crags should be straight forward enough). I don’t mind a bit of hike a bike if the descents are worth it, but to carry up and carry down as well…could have gone for a walk instead.
p.s. I live in the Lakes and have done most known mtb routes round here without problem. I also have a GPS file for this route if you want it.
-DF-