Bikes are still free to go across the Dartford Crossing. You need to check the hours as they are getting a bit sneaky in having a couple of periods when there is no service (call me cynical but I believe it's to cut the number of users to that they can eventually say it's not viable). All you do is ring from a little phone at an office either side of the crossing.
If you're taking your own bike, Dartford to Erith is nowt of a commute although you're unlikely to see a lot of scenery. For a leisure ride, I'd count Whitstable as easily reachable (around 45 miles) or if that's too far, go out on to the Hoo Peninsular there's a really nice pub (Horseshoe and Castle) just past Jools Holland's castle. Best steak and cheese sandwich I've had outside of New York (but it was a couple of years ago)