College doesn’t specifically prepare you for the GRE any more than high school specifically prepares you for the SAT. Pcik up a GRE prep book at the library and take a look at the test. It is only marginally harder than the SAT. There are subject exams for some fields of study, but not all. If your field doesn’t have a subject exam, and you normally do well on standardized tests, the GRE probably won’t be a big deal for you.
I completed my second masters degree through a distance ed program. The learning experience is indeed different from a classroom based one. I missed the live interaction of the classroom, and it was sometimes challenging to communicate with faculty. However, online communications has improved so much since then that your experience is likely to be completely different from mine. It is much easier to emulate a classroom experience electronically now. For someone with a shaky health situation like yours, online really makes sense. You can pursue your studies without having to leave a medically safe zone.
Do the online programs you are looking at offer an option for a summer term or semester on campus? That is something you should ask about. When your health permits, you could go to campus, spend time “live” with some of your professors and classmates, and have at least some of the on-campus college experience.