The reason I ride sportives is that they give me a chance to test myself against myself. I set targets in my head and see what I'm capable of. The fact that there's a safe designed route with varied terrain and loads of other enthusiasts doing the same makes for a day out.
In 2012 I rode Newcastle to London in 24 hours, 300 miles at average riding speed at over 18mph.
The year before a group of us did Coast to Coast in a day, 157 very hilly miles so standard 100 mile sportives are great prep for events like that.