Even though you are fit, you still need to get your muscles used to that amount of walking. Start going for walks in the evening and at the weekend so you are not suddenly starting walking with no advance training.

Also - remember you are on holiday. Don't try and squeeze too much in, and take some time to stop and stare.

And work out the public transport systems. I can't believe that in any of those countries, you do actually need to walk everywhere. What if it rains all day? Are you still going to be tromping through the streets in the rain when you could be riding on a bus/train/tram which takes you exactly along the road you are walking along?