No special access issues
The flattest approach to Mariholtet is from Ellingsrud parking. This is about 6km. The path is very nice for biking which makes the approach a lot easier and faster than walking. There is plenty of opportunity for putting up a tent in the area.
Alternative route is from Østmarksætra Parking. The distance is about the same as from Ellingsrud, but the path is quite a lot more hilly and will make for more a more demanding hike.