Okay, so I've figured out that I have 18 gears on my Trek (2 in front and 9 in back). If I have done the calculations correctly that means my "granny" gear has a 37 gear inch rating.

I'm 62 don't think I'll ever condition into 37 being good enough for hills. What do I need to replace on the bike to get it down to a 19 or so?