The reason it doesn't make sense is because it's like saying, "I'm going to play college baseball, but if that doesn't work out I'm going to play in the MLB." If it's your plan B, please please please have a plan C.
...but ok, path to medical school:
Get admitted to a university. If you completed a 4 year degree in marketing, you can also look into a post-baccalaureate premedical program. Regardless of which route you take, you're going to end up taking, at a minimum, general chemistry, organic chemistry, calculus, physics, and a year of biology. Labs included with those as appropriate. Some schools will also require or highly recommend a course in biochemistry, writing, or humanities, but these last requirements vary by college. Additionally you're going to want to try to find some time to volunteer, possible do some research, and get some clinical experience. While working as an EMT-B (certified is not the same as work) is a clinical experience, it is kinda of weak overall.
After you complete your prereqs, you're going to want to take the Medical College Admissions Test (MCAT). The MCAT is a 4 part exam involving 3 multiple choice sections covering general chemistry/physics, organic chemistry/biology, and verbal reasoning. These parts are normalized and testers are given a score between 1-15 per section with an average score of 7. The final section is an essay portion which is given a letter grade. The exam itself is now computer based (however, unlike the NREMT exam, the MCAT is not adaptive), but expect it to take at least 4 hours (which is better than the 10 hours it took when it was written). The average MCAT score is a 21 (7 per section). The average applicant to medical school is 27. The average score for matriculates is around 30.
Now you enter the application cycle. Most schools will receive around 6000-7000 completed applications, interview around 700, and accept 400-500 for a class of 200 students. Making it to the interview stage at any specific school is a combination of scores and luck. Even if a student has a 4.0 gpa and 45 MCAT (in which case, someone just full filled Washington University in St. Louis's wet dream), it would still pay to apply broadly.
Application for MD schools is done through the American Medical College Application Service (AMCAS), DO schools through the American Association of Colleges of Osteopathic Medicine Application Service (AACOMAS), and Texas schools through Texas Medical and Dental School Application Service (TMDSAS). Regardless, the application process is essentially the same for all schools. The primary application (through the above services) includes your academic information, extracurricular (including work, volunteer, clinical, etc), exam information, and a personal statement. Then most (most don't screen, but a few do) of the schools that you apply to will send you a secondary application that will vary from school to school, but can include additional short answer essays, listing of extracurricular, a picture, and a check for normally around $100.
Now, if you're lucky, you get invited for an interview. Depending on the interview, you will either get accepted, wait listed, or rejected. Some schools will form a second wait list once they fill their class where the people on it are offered a deferred spot in the next year's class. Otherwise, if you aren't pulled off the wait list or are rejected you get to repeat the entire process next year. Almost all schools will accept applications from reapplicants, however some schools will limit the number of times that people can apply. To be fair, this is probably more for the applicant than for the school. It's very easy to sink thousands of dollars (I probably spent $3000-4000 dollars last cycle between primary application costs, secondary application costs, and traveling for interviews) into applying with nothing to show for it but rejection letters.
Veneficus and I aren't being harsh because we're elitist turds who thinks that only a select few geniuses makes it to medical school (to paraphrase a physician on a JEMS Connect thread, 'cream floats to the top, but so do gaseous turds'). The stark reality is that medical school isn't really a "plan B" and the application game to get in is long, hard, and expensive. If you're serious, it is something to work for, and I'll encourage anyone who is serious about medical school. However it's not really a fall back position.