Throughout Europe, the terrain is hilly. In general, you can expect at least 2 or 3 major hills on each cycling day. These may each take about 15 minutes to ride up. Riding hills introduces variety into the day's ride which can be less fatiguing than constantly riding on a level surface. The key exception is the South Holland tour, which is flat.
Similar circumstances apply for the walking tours.
Specifics on terrain can be found on each tour's detailed Fact Sheet. A basic level of fitness is needed. The fitter that you are, the more you will enjoy the tour. One way to judge your fitness is through testing provided by various sports clinics and clubs. We spend two nights in some hotels, and you are welcome to take a day off from cycling or walking if you would like on the second day of each stay.