There already pretty good answers (Even <PERSON> is a greate one!). It's a glorified comment to add a few points.
1. Economics
Food One side effect of a entire population running around is calories needs increase. This makes land even more valuable and food production even a bigger business.
Oil Car and other vehicles can still be necessary for long distances depending of how much a human can endure to run at top speed. Cargo vehicles like trains and ships are still common. Anyway oil consuption ill decrease a bit an can even not be biggest business around.
Shoes A interesting side effect can be high demand for footwear. Running at 40mph all day ill make people buy running shoes weekly. 207
Footwear industry can be a lot larger in this hipotetical world.
2 Culture
WAR Running at 40mph a platton trained to run carring heavy logs can crush gates and break enemy lines with easy. All tatics can change since mobility is a key factor since the first pre historian tribal skirmishes. At that speed spearmens and javelins throwers can be fearsome.
Streets Since cars are more a utility for cargo and long trips streets ill be build for fast running people. That means maybe a grass lane with a danm good drain system.
Transit Law enforcement in transit ill don't need the arise of automobile or the big confluence of horse powered wagons. Each ancient civilization ill create his transit code and in modern days we ill got specialized transit laywers.
Alcohol can be strict proibited for e everyone!
Communication Even in medieval eras communication ill be a lot better between tows and feuds. Also depending on human endurance trips can be a lot more common increasing trade leting people in touch that with the mobility of armies and the need for more land can even "enlarge" a lot the size of the small medieval feuds and nations ill be likely to form early.
3 Science
Need is the mother of invention That's true so steam power => Combustion => Aviation can arise a bit later for example but A increased commnucated humanity can need telegraph early.
Now trying to respond the question:
How to keep stupid people alive
Biology Just make a better peripheric vision. Human hability to driveat 40mph proves we don't need a lot more upgrades.
Mobile Phones Can strict proibited to move and use a mobile device for obvious reasons. In some places they can be entirely banned.
Alcohol Proibited since ancient times! Can even be a taboo
Sorry for the lack of beer & wine guys. 222