100 miles at more than 25 mph on the road is world record territory. Under 5 hours for 100 miles riding solo takes a lot of fitness and/or serious genetics. Under ten hours for a double century is impressive. Under 1 hour for 40Km (25 miles) on a standard road bike used to be what an average Cat 3 would do but the standard is probably more like 58 minutes today owing to better equipment. Do 25 miles in under 55 minutes on a standard road bike, get a Cat 5 license and ride right off the front qnd expect to be a Cat 2 soon.
15mph average on the road is probably typical for recreational riders on average roads.
Hills, road surface, temperature, stop signs, lights, and other factors make average speed comparisons not very useful.
Power and heart rate are better indicators.