I ride mountain bikes still and a change of tyres to High pressure slicks and I can do century rides on it. Any bike can do milage- it is you that has to work up to it.
And on locks- A top rate lock and a top rate hawser to lock wheels to frame to immovable object will deter thieves- but that is all. If they want it- they will get it- but the best you can do is make it difficult for them. Only secure way to lock a bike is to always have your hand or your Butt on it- and that does not always work.
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How long was I in the army? Five foot seven.
Spike Milligan