Oh man, do I know the feeling of running with lead weights - sometimes my feet feel like concrete and my legs are made of wood. For me, it is all about mind games. If I'm on a run, I play games with myself, like "ok, just run to that mailbox, then if you can't run more, walk"... then I get to the mailbox and tell myself "ok, now just to that 2nd lamp post, then walk". Before I know it, I have coaxed myself into running longer - and by making 'micro goals' I often end up running faster and further.
If I'm riding my bike, sometimes the hills seem longer and steeper than normal - I just picture myself cruising down the hill after, telling myself that the short term burning in my legs will be worth it once I'm on the downhill... maybe invoke Jens Voigt and tell my legs to shut up :). Before I know it, I'm over the hill, flying down the other side, and then the miles start to roll by.