I really struggle with maintaining a good cadence. I definitely tend to prefer to mash at a low cadence and have to be really conscious to try and correct it. As soon as I stop paying attention, I find I'm spinning slowly again. I'm sure it has a lot to do with not being in the best of shape, so I find I tire myself very quickly if I try to spin faster. I can go a lot longer at a slower cadence...until I hit a hill, which just sucks all my momentum away and pedalling becomes an agonizing grind.
I think part of my struggle is that my road bike has biopace chainrings, which if I recall correctly, encourage a lower cadence.