Theft doesn't seem to be much of an issue to typical campgrounds and there's no way to effectively lock my tent and other belongings. But I do use a light cable to lock my bike to a tree or picnic table to prevent a spontanious theft of opportunity. I frequently combine hiking and biking by riding to trailheads. Then I try to take the bike back a little way into some woods before locking it so it isn't visible from any main road.

Biking and hiking don't use exactly the same set of muscles, but there's still a big overlap. The main leg muscles are strengthened by both and of course your basic cardiovascular fitness carries over. So I'm not surprised that you noticed that the biking helped with hiking and vice versa. But there are also some different muscles used for each activity as I notice the day after a hard hike if I've only been biking for a long period before.