RE: PUCMM...unless you have trouble studying at a Catholic institution, per se, I can tell you that there are Mormon, Baptist, Pentecostal, Muslim, Animists, Seventh Day Adventists, Jehovah's Witnesses studying there...It is not an institution that pushes the religion. And the language program is not in the church or the chapel.
Regarding language learning, you do need formal grammar studies, and practice. As Chip suggested some night school would be good. Language tapes and on-line programs are also good if you are disciplined enough. Watch tele-novels and beisbol en espa?ol. Fox broadcasts f?tbol in Spanish...anything that you can see and hear at the same time.
That will prepare you for being here in a total immersion situation, where your ear will become attuned to the local variety of Spanish. You can get by with a 1000 word vocabulary. The fluency comes with practice. Therefore, a few months here, without having to do your missionary work, will certainly be to your benefit.
HB