Double the distance, aim to complete with an average speed 10% less.
Your average kCals/km will be the same. Reduce speed by 10% and your kCals/min will drop by a larger difference due to the vehicles CURVED power vs speed curve.
When you move from 200 to 400km, operate the same rule. 400 speed can be 90% of 200 speed.
Example.
100 speed is 13.5 mph, 200 speed is 12.1, 400 speed is 10.9, 800 speed is 9.84, 1600 speed is 8.8 mph.
Seeings as events 1300 to 1800 have a min speed of 12 kmh ( 7.45 mph ) your safe.
When your avearge speed for a 100 minus 10% is lower than 14.3 kmh ( 8.9 mph ), you're not ready.
111% of 8.9 mph is 9.86 mph.
Big Tip. Don't go rushing off from the start of your 200 at the same pace as you rode the 100, or at the pace of more experienced riders. Stay at the target pace.
You'll be fine,,