Is there alway a long way to run between gates?
Which airport will you be at for your layover? As Nick & Cindy said, in general 45 minutes ought to be fine. However, with some airports that may be a bit of a challenge depending on where your departure gate is relative to the arrival gate. In some cases, you may need to go from one concourse to another, which could involve catching a train, or a lot of walking.
Listen to the flight attendant's announcements as you are getting close to landing at the layover airport. Usually they will provide "connecting gate information", that is, they will tell you which gate you will be arriving at, and then list the upcoming departing flights and their gate numbers. The airline's magazine, which is usually in the seat pocket, typically has maps of the hub airports, so you can find your arrival gate, your departure gate, and get a good idea of whether you need to stroll ... or sprint.
I've had many, many layovers, and while I've had a few sprints that ended with the plane door closing right as I squeezed through it

, I've never actually missed a connection, so don't worry about it. Just be prepared not to dawdle if your first flight is running at all late. Oh, and don't hesitate to ask the agent that is usually waiting at the exit from the jetway if you need any assistance in determining the quickest way to reach your connection.
Good luck!