Starting up from Troutbeck to Thornthwaite Beacon is also entirely legit. I think that's the best route plan, with the option of dropping back down to Hartsop for a shorter day out, though you do have to ride over Kirkstone on tarmac at some point in the day. SFB will be back in a minute to point out that he prefers going straight up from Hartsop 😉
As he said there are various options at the far end – look out for the left turn at the tumbledown ruins on the way up Loadpot Hill, the singletracky traversing path is more fun than flogging up and over (it's also the theoretically legal route).
And +1 for the direct descent from Arthurs Pike over White Knott.