Not necessarily! From the South Bank to Holborn Viaduct is about half a mile and it is definitely quicker to take the train.Blackfriars to City Thameslink. By the time you walk to the platform and get a train, you could have walked directly there in 5 mins.
That's quite easy though just change at Abbey Wood. The best pointless journeys include long and complex routes well out of the wayWoolwich to Woolwich Arsenal is one.
In the area there is Edgware Road (Bakerloo) to Edgware Road (sub surface), changing at Baker Street or Paddington but quicker to walk.Paddington - Lancaster Gate: A five minute walk, but requires a change at Notting Hill Gate or Oxford Circus if using the tube.
Marylebone - Baker Street: Another one stop tube journey that's just as quick to walk most of the time.
Quite a few options there depending on which direction you start off in. One that springs to mind is St Pancras - Stratford International - Stratford - Farringdon - St Pancras but there will be many others.St Pancras (upstairs) to St Pancras (downstairs)
When the Kent stoppers return you could do it St Pancras-Lille Europa-Ashford International-Tonbridge-Sevenoaks-St PancrasA shame that we never got the north of London Eurostars. That would have been a good route from the international to Thameslink platforms at St Pancras.