Probably a decade ago, I hiked the Kolinpolku Trail in Finland together with a friend in early spring. The trail was awesome, the decision to go very early in the year was bad since we had partially still a lot of snow making progress quite complicated.
The trail is noted with different length depending on where you search. The map below talks about 70 km (foto taken on the trail) while various finish websites talk about 63 km and 3-4 days of hiking to get it done. But also 67 km is possible, so it obviously depends on where you search. But in case you go in summer, you should easily finish this one in 4 days maximum.
We were flying into Joensuu and started hiking up North directly from the airport. This is not perfect but is possible and you don’t need to take care of any public transport. So all in all this is a nice trail which was completely empty in the early year with partially pretty awesome weather. Solely the level of snow and ice was partially a bit too high.