I mainly ride offroad and when I tell roadies that I only manage 30 miles or so on a Sunday- They look at me with pity. In Fact, now I have a road bike- milage means nothing to me. 50 miles on a road bike on tarmac gets very boring and normally has to have a couple of cafes in it to break the monotony.
Now the height of the summer offroad brings another peril to me and that is (over) confidence. All my major offs have been at high speed on a Rock Hard trail and I don't bounce as high as I used to. In the winter I still fall off but the biggest danger is 6" of mud down the neck.
There is a technique to offroading on Mud or loose soil and somehow I seem to have it. Just wish that the masochist in me didn't try to find the wettest puddles or the deepest mud about. Poor picture I am afraid but the rear wheel of the Tandem is in one of the ruts on our trail to the hills, wheras the front has just got out of it. Just shows what a little bit of rain will do on our local MUP's
As to miles being different- They are all the same- its just that some take longer to travel.
Link to my winter preparation to show you are not alone.
http://www.bikeforums.net/showthread.php?t=239968