To ride i mean.
From Rochdale to just west of Cockermouth, probably aim to go up the coast road in Cumbria so head towards Barrow & turn North.
Is two days a realistic prospect (B&B overnight) or am i asking for trouble with the terrain etc?

On road? Big day ride or comfortable two dayer I’d say.

It’s a long way- getting on for 150 miles, with a lot of up and down. Don’t go near Barrow but cut across the top of the peninsula from Greenodd- Broughton. If you are feeling brave there are bikeable fell roads which save you distance but with some pretty hard climbing (e.g. Ulpha Fell and over to Eskdale). I lived in south Cumbria for ten years, so I have the scars.

I’m buggered if i’m riding over Ulpha after all that distance! Having said that, the route into Ulpha from the South is gorgeous.
Once into Eskdale it would be fairly easy though, hmmm decisions..

Where would be best to stop overnight before hitting the Lakes?

I did it once, on the way to a fell race out west. Not a good warm up!

Where to stay- Grange/Lindale/Newby Bridge would be slightly over half distance.

Newby Bridge is a good idea!

Quickest way would be go straight through kendal, windermere, ambleside, grasmere, dunmail raise, keswick.

Losd of options though, could do Honister Pass, a nice scenic route.

The Coastal route is probably the longest I’d have thought, turn off at Greenodd. You can miss bits of it out if you go over Corney (brings you out at Muncaster) or Birker (brings you out at Gosforth) fell. Just make sure your brakes are working.