I wouldn't recommend walking to Dafni from Kalamaki at all, as it is all hills, and no beach, so it is dangerous and impossible to walk to. You have to walk up into the mountains, which is very difficult going. However, you can swim from Kalamaki!!!
Dafni Beach used to be really beautiful until they deliberately trashed the beach with rocks, stones, rubble, and built loads of illegal building, tavernas and car parks, basically to put off turtles. They have done so much damage. Bulldozed the sand dunes to turn them into a sun bathing area, complete with a lawn and tropical palm trees. Dafni also used to be the second most important nesting beach in Laganas Bay for the Loggerhead turtles, and now nesting is seriously reduced, almost disappeared. (Sorry, you might have guessed, but I don't like Dafni!)
The best place to walk is at Gerakas. You need to drive to it, but you can do a circular walk from the beach, up into the hills and Vasilikos forest, down towards Porto Roma and then back again. You can either get the guided trail from Friends of the Ionian, or if you are on the island this summer, come to Gerakas, and get a self-guided trail from the Earth Sea & Sky Info Centre just up from the beach.