The cabin is completely idylic and exactly as described. You're close enough to the main path to run down to the lake for a swim but far enough away that it feels completely private. The lake is small enough to swim accross if you are confident and by mid afternoon was the perfect tenperature to cool off (not suitable for skinny dipping as there's always the chance of a cyclist or hiker passing by).
The kitchen had everything we needed to prepare and eat the food we'd brought. The cabin is close enough to the city that we we're able to bribe some friends to come for a day trip mid week with extra supplies (wine) in return for apple cake at the gorgeous Frognerseteren which has a suprisingly good selection of vegan cakes.
Trails around the woods are easy to follow as each route is maked with coloured markers so you do not have to be constantly checking your map as you follow a variety of circular trails but do make sure you've looked at where you're going as some of them cover more difficult terrain than others. Waterproof boots are a must.
Ullevålseter is a good starting point for a number of routes and serves lovely cakes and sandwiches (they also have vending machines outside if you miss opening hours).
If you're looking for a getaway where you can get your hands dirty but also have the luxury of reading your book by the fire or lounging in a sauna as long as you want this is the perfect place.
There's no wifi but the 4g coverage is excellent if that is a concern to you.