How to pace Ride100?

Think of your fastest pace and ride at 60 - 70% of that and you'll be fine. I reckon 6 - 6:30 is going to be your time. Start a sweepstake!

Depends when you start as there can be backups in flow. Just ride with a comfy pace, so you can speak easily. Eat plenty per hour drink plenty per hour. Ride with a cadence of >90rpm. This will stave of fatigue in the legs till much later. Don't grind it out-spin away. Enjoy the day. It can be a fast course if you get a clearish run. Also draft whenever possible there will be thousands of rider to draft with

It's really hard to answer your question. I think if you rarely ride 50 miles you will find 100 miles hard and my advice would be to take it really easy for as long as you can at the start, to stave off the point at which your legs give out on you. This would apply more if you were just trying to do a century by yourself.
However, Ride London is a unique event. The closed roads and the people cheering you on will give you a boost. There will be bunches of 10+ riders passing you all day long, so if you can get in a bunch that are riding at a pace you can sustain, that will massively help you hold a good average.
I did it in the first year, I think under 6 hours, and also the year of the storm, where I averaged 20 mph - maybe because I just wanted to get it finished!
I found the route pretty good, the start all the way through London is brilliant - flying along the three lanes each way roads at about 23 mph with other bikes all around some slower some faster ignoring red lights and going the wrong way round roundabouts with the public cheering you on is an unforgettable experience! I seem to remember struggling with it after Weybridge before Newlands, just a bit suburby before the interesting stuff.
Don't underestimate Leith, I think it is a reasonable challenge and as others have said narrow enough to make finding room difficult. It is quite long and has steeper parts to make you work. Wimbledon too can hit you hard, a long way in and not close enough to the finish to just breeze up. The last stretch as you get back in to the city with the Embankment and then people banging the boards up the Mall is fantastic too!
Have you thought about eating and drinking? The official food stations at Hampton Court and Newlands were mad-busy when I did it; if you can take enough food and drink to last you until the last third or quarter you'll probably be better off.
Lastly, if it is hot and sunny that will really hit your ability to go fast for a long time.
Why not treat this year as a learning experience, and then enter next year with more of a plan?

Are there any other points of reference? Have you done any other parts of the Ride London course...? Any of the climbs or comparable climbs? What's your longest ride so far..? Do you do any other endurance sports?

Without other information it's impossible to call, but I agree with what alan above has written. I'd add that if this is your first 'long' ride, forget targeting an average speed: focus on enjoying the experience, breaking it down into smaller chunks, and just getting round. I know how boring that seems, but you've got to start somewhere.

1300m of elevation gain over 160km of riding is a flat course. If you regularly ride these sort of distances then 5.5 hrs is very possible (if you have an early start). There are only three 'climbs' of note: Newlands Corner (good feed staion at top), Leith Hill and Box Hill. Leith has the steepeest sections (8%) and Box is the longest (2.3km at 4.5%). None of them are any real challenge to a competent rider. After Box Hill you should be able to average 20mph for the rest of the ride. There is one little bump at Wimbledon just to let you know you have ridden almost 160km... If you have an early start you will have fewer numpties to deal with. Anything after 8:00 am and there is no way you will keep a good average speed up - there are 30000 cyclists expected... The main issue is on the three hills with many coming to complete stop and causing massive backlogs. My first time in 2015 I had a 6:15am start and finished in 5:20 with stops at two of the feed stations. Last year I started at 9:00am and it took over 7 hours. However, my average speed after Leatherhead was 32km/h into the Mall. Good luckand enjoy it.

I'd suggest 5.5hours is ambitious depending on your general fitness. I work in mph rather than kmh but you're looking at an average of 18mph which is pretty tough going when you get beyond 2.5 to 3 hours of riding. And if the furthest you've ridden is less than 50 miles then survival is a big part of it - especially when you go north of 80 miles. There's also eating, drinking and a slow start to consider, plus the dickhead racers mixing with the barely competent.

You also ha e to consider that you don't do hills, at all. I don't know the course but if there's anything above 10% or longer than a km or 2 long then they will nobble your speed significantly.

I would be aiming at averaging 15-16 mph for the first 4/5 hours and then up it if you're feeling good.