You can't convert miles to hours directly. You can, however, determine how many hours it takes to drive a certain number of miles, and you can also compute the number of miles you travel within a specific number of hours. Both calculations require a determination of speed, which is a number that you derive by dividing the distance traveled by the time taken to travel that distance.
If you're traveling in a line from point A to point B, determining your average speed is simple. You measure the distance you traveled (or you read that number from a map) and you divide the distance by the time it took to traverse. You can measure distance in miles, feet, kilometers or any other unit of distance, and you can measure time in hours, minutes, seconds or fractions of a second.
Suppose you travel a distance of 100 miles, and it takes 1 1/2 hours to do it. Your average speed is then 100 miles divided by 1.5 hours which equals 66.67 miles per hour. When calculating miles per hour for distances that take only minutes, you convert the number of minutes to fractions of an hour. For example, suppose it takes you 15 minutes to travel 6 miles. The time elapsed is 15 minutes divided by 60 minutes = 0.25 hours, and your speed is 6 miles divided by 0.25 hours which equals 24 mph.
Speed and Velocity
Despite the fact that people often use the words speed and velocity interchangeably, they aren't the same. Velocity is a vector quantity, which means that it has a directional component. This is an important distinction for scientists, and it's also relevant to anyone traveling from point A to point B on a winding road.
When you're traveling directly toward your destination, your speed and velocity are the same, but when the road bends to the right or left, the velocity changes. The speedometer may indicate a uniform speed, but each time you veer off course, you're approaching your destination at a slower rate. On most real roads, you must take these changes of direction into account to get an accurate relationship between the distance between two points and the time it takes to travel between them.
Speed Is Rarely Constant
Another complication affecting the relationship between speed and distance on a typical road is the fact that speed is seldom uniform. During your trip, you'll have to slow for curves and slower traffic, and you may even run into a traffic jam. On the other hand, you may travel faster than usual on straight stretches of road.
It's impractical to take these fluctuations into account. They sometimes cancel out, but more often they don't. Because they affect the elapsed time and your average speed, they may lead to an inaccurate relationship between the distance traveled and the time taken to travel it.
There's an App for That
On some paper maps, you'll find charts listing the distances between major cities and the time it takes to travel between them. These charts base the distance/time relationships on the speed limits, and they take into account the characteristics of the main road you take. You can use them to convert between miles and hours, but there's a better way.
Smartphone map apps are more accurate because they monitor traffic, which can sometimes make the difference between a 2-hour trip and a 6-hour one. Traffic conditions are constantly changing, and driving time estimates on smartphone maps change with them. Some apps even suggest alternate routes to your destination that, due to current traffic conditions, will get you to your destination faster than the main route.
If it's important for you to know how many hours it takes to drive a certain number of miles, your best bet is to download a live map app and use it. Just be sure to pull off the road before you check it.