AHA! BP meds are probably limiting your HR. Maybe. I know nothing about them but if they're beta blockers then yup.
7k feet of climbing is a lot of damn climbing. 120 miles is a lot of damn miles too, I've never done it.
Get miles in your legs, lots and lots of miles - I would advise >400 a month between now and your event and you NEED to do a ride with that much climbing before your event (doesn't have to be 120 miles). I can't remember what you said you were doing in your other thread but your legs will appreciate the training now.