Yes, it is true that you can drive 4,500 miles in North America, from Miami to Fairbanks, and English will be the official language all along the way. But you can drive 7,500 miles south from San Diego, California to Punta Arenas, Chile and Spanish will be the official language all along the way! (Actually it's 6,537 miles as the crow flies, but I said 'drive'! Incidentally it's 3,963 miles from Miami to Fairbanks, as the crow flies.)
In fact, for the Americas, in terms of the numbers of native speakers, the principal languages are Spanish, English, Portuguese and French, in that order! So, unless you go to Brazil or Haiti or Quebec, if you speak both English and Spanish, you can get along quite nicely. For the entire world, Spanish is the fourth most-spoken language, after English, Chinese and Hindustani. It is widely spoken in Europe.
But there are other reasons to learn Spanish:
1. Talk with your neighbors. There are 30+ million or so people in the U.S. who speak Spanish. Once they were concentrated along the Mexican border, in Miami and New York. But now they are all over the U.S. It is useful to be able to communicate with your Spanish-speaking neighbors, many of whom don't speak English.
2. Learning a new language is good for your brain! People who stay active mentally, taking on new challenges, in addition to getting more out of life, are less likely to become afflicted with Alzheimer's Disease or other degenerative ailments. Brain cells are like muscle cells in that if you don't use them, they get lazy, atrophy and become less healthy. Learning a new language forces your brain to grow new cells or find new neuron paths among existing cells. That is good, means you will live longer and enjoy life more.
3. Travel will be more fun! While you can visit Mexico, Panama and Argentina perfectly well without speaking Spanish, you won't have nearly as much fun or get nearly as much out of it as you will if you do. Speaking Spanish will let you explore on your own, it will open doors for you that most tourists don't even know about!
It lets you get out of the English-speaking enclave (The 'Gringo Ghetto'!) and go where the real people are, where the culture, the shopping the dining, sports events and other activities are.
It will also save you a great deal of money. If you can shop in stores, stay in hotels and eat in restaurants where only Spanish is spoken, you will generally find that the prices are much lower.
4. It will help with your work. Being able to communicate in Spanish gives you a competitive edge over those who cannot. You can talk with the cleanup crew, with contractors, customers or clients that don't speak English.
5. It's a cultural experience. First off, it will help you understand your English better. Latin was the official language of England until the year 1250. Many English words have Latin origins. Learning the Spanish version of those words gives new understanding of your English.
You can get a different slant on the news by reading newspapers and hearing the news in Spanish. Seeing events through the eyes of the Spanish-speaking world will increase your cultural understanding and appreciation.
Being able to communicate with people who grew up in another culture gives you new insights into how they think and how the world looks to them.
6. It's fun! Whether it is just from the pleasure of taking on a new challenge and making progress doing so or the fun of watching your ability and understanding increase, learning another language is a pleasurable experience.
7. It's easy! Forget the drudgery of boring classrooms! This is the 21st century! Get a digital media learning course such as Rocket Spanish. It's not expensive, is in mp3 format so you can download to your iPod or burn CD's and listen to it any time and place you want. Repeat lessons at your leisure, go fast or slow. You are in control. You have complete charge of your learning process.
In Summary: You should consider learning Spanish because it can be useful, fun, is a culturally broadening experience, exercises and conditions your brain, equips you to better deal with the modern world and makes you a better citizen and person. With English and Spanish you will be very well equipped to deal with the rapidly shrinking world of the Americas. And a cost-benefit analysis will quickly show that you will get a great deal more out of it than what it will cost you!