depends on the program you end up at.
you can get an MD internship approved in lieu of the DO internship by using AOA resolution 42. the vast majority of these are approved by the AOA.
Most of the time the intern year is rolled into the total time of the training. Most specialties don't add on extra time to compensate for the internship because many of the rotations you do as a DO intern are the same kinds of things you'll be doing as an MD PGY1 (same thing as intern, but the MDs don't really use the term anymore).
If you match DO, you cannot go through the MD match for an equivalent program. ie: if you match DO for a FM residency (PGY 1-3) you're out of the MD match. but...if you match for a DO internship only (PGY1) you can still participate in the MD match for a PGY2 spot (so that you don't have to reapply the next year). There are specialties that expect you to do a separate internship & then join the residency as a PGY2, so this isn't an odd deal.
virtually all FM residencies will be a shoo-in for resolution 42 approval. In fact, many are pre-approved by the AOA even if they aren't a dually-accredited program.
Overall, don't worry about match issues now, DoctorMom. You'll have many options an won't be looking at any extra training time.