Great Langdale to Scafell Pike is a very nice, well trodden route though I wouldn't have thought it was quite that far. If the weather is good and you are used to hiking in Snowdonia/Dark Peak type areas you'll have no problems and a lovely day.
I'm assuming Mickleden, Rossett Gill, Angle Tarn, Esk Hause, Broad Crag, Sf P.
I wouldn't do Scafell Pike and Helvellyn on the same day by any routes, you are going to spend the whole time watching your watch instead of viewing the view.