I am not sure about your particular event, but most bike events have sag units that will pick you up and transport you in case you stop. But I think you can do it. Just pace yourself and make sure your nutrition and fluids are adequate and even if you do not finish the ride, you will gain incredible confidence at what you did do and will be better prepared next time. My husband and I are trying to train for the Florida MS150 next May. We were looking at the suggested training schedule for that ride which is 150 miles over two days and the training schedule for that is only 8 weeks. We are starting now and getting in a variety of grades, elevations (which is tricky in flat old Florida!) and increasing our total miles each time we ride. I am also overweight but hopefully that will change with my increased activity. Good luck to you and enjoy that ride!