Hey, if I can ride 72 miles around Lake Tahoe, at 55 (after having taken up cycling a year ago and coming from flat, near sea-level Dallas) ... and I did, there is no reason that you cannot do this. Take your time - don't get off the starting line at a super fast pace. If you need to, as noted above, break it into multiple short rides between rest stops. Mentally, it's a lot easier to think about having a 10 mile ride (to next stop) than 70 miles left to finish. Like all the advice above - don't linger too long at each rest stop, but take advantage to refill bottles, stretch, eat something easy for energy. Then, sit up and pedal! Have fun!