I've never been a heavy trainer but wouldn't look at doing the Fellsman off nothing longer than 20miles / 4hrs
Obviously training runs / events would be a run round the Three Peaks, Haworth Hobble and the Calderdale Hike (might be too close now, date-wise for the actual event). I'd be looking to do at least one 25miles / 5hr run per month between now and the event if I was you. LDWA events make great training runs