Punching boards can be a good way to train by yourself; never liked heavy bags though I have used them and focus mitts can be helpful as well; but, in the end, RobP summed it up very well. +1
Contact training doesn't have to be unrestrained to be valuable... use boxing gloves and headgear or limit contact to head. Particularly for the latter you need decent partners to get the most out of such training and beginners rarely have the courage or the experience to restrain themselves and still do necessary things like "aiming" and delivering some force the right way as opposed to "I'm going to tear your head off cause it's easier than taking my time and doing it in a mechanically sound way."
Learn to aim, learn to hit the same target repeatedly and keep in mind that hitting a moving target with skill [even in playful training] makes both objectives somewhat more realistic than punching the air or practising your "fast hands" against stationary targets.
Get all this ugly stuff out of the way and someday you can learn the real secrets of New Age Kung Fu - practise endlessly by yourself with a noble and lofty look pasted on your puss, preferably while wearing an antique outfit.