Hey Donna,
Look at it this way....
The 25 mile ride you already know you can do, so it would not be a challenge. But look, you don't 'have' to finish the 50 mile ride- just do as much as you feel able to! It will likely wind up being more than 25 and maybe less than 50....and so what? Just get out there and enjoy the ride and ride until you get pooped out. The actual mileage will not be nearly as important as the effort you put out in doing the ride. Remember, you will also be inspiring others who are struggling to do the 25 mile ride and maybe can't do that yet. They look up to people like you.
Be proud about whatever mileage you can manage- what matters is that you are DOING it and are trying your best.
Be sure to eat a good breakfast- oatmeal, fruit? and yes stop midway and eat some energy food and carbs and drink lots of water all the way along.
Go for it and don't beat yourself up about the actual number of miles. Have fun!