Another positive recommendation for Lakeland Trails Coniston Marathon... Did it last year - great route / great organisation. Course gets harder as it goes round!
A few others not yet mentioned...
Meon Valley Plod - cant rate this highly enough... 21 miles but hard as nails. Well worth a look.
North Downs Way - my local!
Three Mole Hills - mixed terrain but takes in Box Hill and is very well organised.
And one I havent done but plan to next year...
Pen-y-Fan (The Fan Dance)