50 to 75 physically shouldn't be that big of a deal (less maybe little extra soreness to your backside when you finally get out of the saddle ) as long as nutritionally you're ready for it. My local cycling club has about 20 members riding in The Flatest Century Ever ride in SE Mass and RI next weekend, and as such the regular group rides that the club has been doing have been increasing in their mileage over the last few months. My cycle club is very active with plenty of e-mail and facebook and club blog updates happening almost daily, and the number one tip when long distance endurance rides are being discussed is nutrition and hydration (pre-ride, during the ride, and post ride), since unless the last portion of the ride has some "Tour De France mountain top finish style" endings to it, physically you should be ready for it (as long as you still have enough "gas in the tank")
For myself personally my regular rides are in the 30 to 40 mile range, and i've become quite used to the demands that rides of those lengths place on my body from a hydration and nutritional standpoint. What I've found when I plan on going for a longer ride (50 miler, metric century, etc) is that I need to force myself to increase the amount of both fluids and nutrtional supplements (My personal favorite is Hammer Brand Gel Packs) over what I would usually take in over those 1st 30 to 40 miles. It took a few times for me to have significant "bonks" out past that 40 mile mark, for me to realize that I needed to "fuel" myself more, at an earlier point in my ride to enable myself to go the extra miles/hour(s) in the seat without any significant changes in my performance in the saddle.
For example, one of the usual group rides I do is a 35 mile loop that ends up typically at about an 18mph average. In those roughly 2 hours in the saddle, I'd typcially take in 1 to 2 24oz bottles (depending on how hot/cool it is outside) of a protein/carb based sports drink (I personally like a product called Generation UCAN - it works very well for my body) and then at about 2/3rds of the way into the ride, shortly before there's a 1.25 mile climb with a 7% average grade, I'd take in a Hammer Gel Packet, and that overall combo would keep me feeling good for the entire ride. If i'm riding say 50 or beyond, what i've learned that I need to do to keep my body going is that in that same say 1st 2 hours, I need to be drinking between 2 to 3 bottles of my same sports drink and take in a Gel packet about every 12 to 15 miles to enbale me to maintain the energy levels and avoid the dreaded "bonk" I then maintain that liquid + gel pack intake ratio all the way to the end of my ride, every now and then if it's really warm and humid out do have to increase my fluid intake over that rate. I've also learned that for a long ride (4+ hours for me) that if I don't feel atleast some need to pee by the 1/2 way point of the ride that I need to increase my fluid intake rate, and if I've managed everything correctly, at the end of the ride I'm feeling an equal need/want to eat something, drink something AND find the bathroom!
It took me bonking a few times to realize just how close my "usual" hydration and nutrition intake was getting me to the "bonk zone" and learn where my bodies typical endurance limit was. Now that I know where my "bonk threshhold" is, I can truely say that physically there isn't a heck of a lot of difference between a 50 mile ride and a 75 mile ride.
So just experiment a bit with different types of sports drinks and nutrional supplements before hand so you'll reach mile 75 feeling good!