Similar to jppe, I live at around 150 ft of altitude and try to get to Ride the Rockies every summer (have done 5). I fly out a couple of days beforehand and spend time with the grandkids--playing soccer and keeping up with them. Then I go on the ride and pace myself. Have not had issues with altitude--and like jppe says, we have done some pretty big climbs. Hydrate a LOT... if you are not going to the bathroom every hour or so, you are not drinking enough. This at least for the first day...that helps a lot with the altitude. If you can, sleep low, play high.
train safe-