Casey, a couple of things on what others said...

This isn't an old Airfix kit, but a new tool 2011 product. The issue you have with the tracks is partly due to the fact that you haven't cleaned up the mould line that runs all the way arround them, right down the centre of the track. (This is due, some may not be aware, to the tracks being once injection moulded piece, not floppies at all.) That is quite tough to clean up as you don't want to damage the tread, though of course you don't need to do it where it contacts the ground, so it's not as long a task as it may at first seem.
The tyres - there's been discussion recently here about this (look for a thread called something like "the Ken Abrams method") and as James says, paint the wheel in the base colour first, then with your black well thinned, let it run around the rim. If it doesn't quite cover properly, do another coat - but thinner paint will track around the wheel / tyre rim more easily.
I have to say I think you've done well considering you were using the paints in the kit. Did you prime it first, or was that painted straight on to the plastic?
You may find this thread of interest, the same kit:
Airfix Cromwell IV - a very nice build by Michael Johnson.