Keep in mind the off the road rides organized by the knmv require a (day)license, which you can buy for a whole year or just for the day. There are some requirements for the bike. It needs road legal tyres, good brakes and stuff (which is logical)
Those rides are great fun and quite a bit of off-road, because of the big amount of riders the tracks can become quite rough! (which is good)
Other rides are organized by clubs or individuals (like by people from this club) and those are less rough, but maybe more distance.