Honestly, not hard at all. The major hurdle is getting through one. Postbac programs are good $$$ makers for schools. Get paid tuition, don't give out a degree. Not that that is inherently bad - schools need revenue too. Just something to keep in mind. They will accept solely on the basis of whether they think you will do well enough to proceed, that you have a reasonable chance of going to med school. They try not to pick people who are not serious and/or not prepared, but they do anyway. That is why 50% or more drop out of such programs.
Getting in is easy, getting out is the hard part. Where have I heard this before?
Best of luck!