TomB +1 but with a slight modification and a bit more specific… From Windermere head over the High Borrans to Froswick, above Troutbeck/Kentmere, to gain High Street then walk all the way in to Penrith, nevermind in to Keswick and then getting an (expensive) bus ride! Alternatively, for the early stage, head north through Troutbeck, gain Wansfell, over The Hundreds to Kirkstone Inn then gain High Street via Stoney Cove Pike… If two single train fares might be a bit much the other option is to do a loop from Windermere such as hit High Street, drop down to Howtown, get the steamer down Ullswater to Glenridding, o/n in and around the village, head up on to Helvellyn in the am, then south over Fairfield, via Hart Crag and Red Screes to Kirkstone, The Hundreds to Wansfell then down through Troutbeck back to Windermere.
The only thing I’d say about AlisdairMc’s suggestion is, how early do you need to be back in work on Mon as the Sleeper doesn’t get in to Euston til around 0730/0800 – I used to travel to Inverness on the Sleeper regularly to visit my wife when she was based at RAF Lossiemouth. It’ll also be circa 150 quid for a return, so depends how flush you feel, whereas a return to Windermere from Euston’s about 95 quid.