I go in at Roehampton Gate and go round anti-clockwise. Sawyer Hill is long but not particularly steep until the last minute. The view of London from the top is worth pausing for. The next challenge for me is Dark Hill which is very short , a bit bendy, overhung by trees and a bit brutal for me personally. I tend to stop for an expresso from the cabin in the carpark at the top of Broomfield Hill before gripping the bars tightly and gritting my teeth for a fast, illegal, bendy decent of Broomfield Hill. I've never managed more than 34 mph. Ronnie Pickering types often pass me a good ten mph faster. It's a bit daft really. People get killed there.
It's a great eight(ish) mile circuit in pleasant country....and the deer are fun to look at. BTW, look out for the kamikaze badgers when it gets dark.....
Edit: In the winter, cars are locked out quite early in the evening but walkers, cyclists, skaters, runners and other oddballs have access all through the night. It's a magic place to ride in the darkness and the peace with just a glimpse of herds of deer in as they are lit up by your front light. I can't remember the car free times, but it's something like from 5pm in December. It'll be published on the Royal Parks website. Give it a go.