If you're riding the 26 mile route, you'll approach Parliament square just after mile 13 – and if you're on the 52 mile route, it'll be just after mile 34.
Don't get too concerned – there will be marshals along this section to make sure you're heading the right way!
As you pass the houses of Parliament on your right hand side and approach the square, move over to the furthest-left lane. You'll be able to leave the traffic lights first, and be able to move carefully across to the cycle lane that runs all the way around the inside edge of the square. Stay in the cycle lane until you reach Bridge Street. (see arrows below)
Once you exit the square onto Bridge Street, continue onwards to the set of cycle traffic lights. Here you'll turn left onto Victoria Embankment, with its new cycle-superhighway that runs all the way down the river (see arrows below). You'll now have about 14.5 miles to go!
Hopefully that's all clear – see you on Sunday!