You should also look into Doris Dukes fellowships if you want to do clinical research instead.
It's important to be realistic with yourself and keep in mind what you want to do in the long run. In my case, I was doing basic science in undergrad and med school (including an extra year of funded research), but realized I wasn't happy and swapped to clinical outcomes and trials in residency. I felt really behind compared to others who had been doing clinical or health services research.
Several things:
1. IM is a buyer's market. Just because you are at a mid-tier school doesn't mean you can't get into a top IM program.
2. Keep in mind that most cards programs will heavily favor their own IM candidates, so you want to look for an IM program that has a strong fellowship. I think there is less of a difference between mid- to top- tier programs as these boards sometimes makes it seem. With that said, some "mid" tier IM programs are linked to "top" tier fellowships (don't want to name any in particular, since it would probably start a debate), so you might want to aim for one of these. When you get there for IM, seek out a cardiology faculty member you like and develop a good relationship with them. It will make a huge difference when you apply, whether internally or if she/he is willing to make phone calls for you elsewhere.