There is a bus from Mariefred to Åkers Styckebruk that goes from town, or one could take the antique steam train (recommended!) to Läggesta and take the bus from there. I think there is a package price if you already took the S.S. Mariefred steam boat from Stockholm!
I examined the hiking possibilities if one was to have common stops, but the nicest places to camp are not exactly accessable, if one was to do a bit of bushwacking it would be possible, I don't know about the time it would take. The nicest features of the canoe trip is that the lakes are in between high ridges, which makes it complicated to hike alongside them.
How hard are you guys thinking of paddling? Åker to Gnesta in two days sounds like a hard pace. (27km). I am not an experienced paddler (yet, I got a few months to train), how many stops/overnights are reasonable? I'm more of a hanger than a paddler
Would like to spend as much time possible swimming and camping/eating/hanging.