Appreciate any thoughts on doing this from Oslo in one day. I understand it would be first train out of Oslo to Myrdal and last train from Voss back to Oslo.
Also on a separate topic, any hikes near Oslo that people have enjoyed. Assume very fit couple in 50s.
NIN usually is taken from Oslo to Bergen and either spend a couple of nights along the way or go straight thru to Bergen in one day and spend some days in Bergen before returning to Oslo. It may not be impossible to go from Oslo to Voss and back in one day, but it will be a very, very long uncomfortable day of train transport.
Did not do traditional hikes outside the city, but the city itself is spread out and quite walkable to enjoy.
The highly scenic fjord area targeted by the NiN route is much, much closer to Bergen than to Oslo. It's easy enough to do a round-trip to the area around Flam from Bergen (though you miss the very good scenery between Myrdal and Oslo). I don't know that the schedules will cooperate to tackle the whole thing while based in Oslo. I wouldn't want to attempt it myself. I'd try to shift my itinerary around to allow an extra day. Otherwise, I'd postpone the Nutshell journey for a later trip. Norway has a lot of great destinations in addition to Oslo and the Nutshell route, so it's not like you'd be returning to Norway just to take the NiN excursion.
Yes, there is a version of the Norway in a Nutshell that can be round trip from Oslo in 1 day. You can book the package with FjordTours or DIY.
Yes, it will be a very long day. Departure around 6:30 am and return to Oslo around midnight. Depending on the availability of the components (trains, bus, ferry) you may need to travel from Oslo to Voss first , then bus to Gudvangen, ferry to Flåm, train to Myrdal and train to Oslo.
Keep in mind sunset times if you're traveling early or late in the season.
If you can possibly do an overnight in Flåm or Aurland, you could visit Stegastein Viewpoint for spectacular views overlooking the fjord and do some hiking in the area.