This thread has made me realise how few services I've done the full length of in 1 go!
Kirkby/Headbolt Lane - Liverpool Central, Euston - Manchester Piccadilly, Manchester Picc - Hatfield, Rose Hill Marple, Sheffield, Chester, Stourbridge Shuttle, Inverness - Kyle, Aberdeen, Edinburgh. London Liverpool Street - Southend Victoria, Waterloo & City line, Leeds - Ilkley, Bradford, Skipton, Knottingley, York (via Harrogate), Doncaster, Ilkley-Bradford, Liverpool - Wigan, Edgware Road - Wimbledon
Probably missing a couple I didn't think of
I like to split up journeys wherever possible (and I avoid changing at termini and usually use small stations if possible to tick them off).
And maybe counts:
Chester - Liverpool Central