Well Caroline, you have plenty of time to get in shape for your ride.
50 miles is not that hard, unless there is a great deal of climbing of course.
1) In order to get in shape, you need to do aerobic exercise as many days out of the week as you can. For example, if you ride on weekends, try to get an hour on several of the weekdays. Bicycling is best but windtraining or spinning works too. Even fast walking will work pretty well. But basing your conditioning on exercising only on weekends is pretty tough.
2) Another thing to do is keep a rough total of your weekly mileage and try to increase it by no more then 10% per week.
3) Work on various aspects of cycling: pedal stroke (rpm) many rookies have a very low cadence and need to get it up over 80 rpm. Cornering, braking and other bike handling - doesn't have to be anything extreme. Riding in traffic - learn how to ride in traffic safely - Forester's book "Effective Cycling" is still the best treatment. Learn basic road repair - like fixing a flat. It can come in mighty handy.
4) Winter is coming up so depending on where you are you might have trouble training. You can get involved in spinning, windtraining, walking etc rather then losing your conditioning ove the winter.
5) Now do other exercises help bicycling performance? Well, the best way to improve cycling performance seems to be cycling.
Once you can average 15 mph, 50 miles should not be too hard. It is just 3 hours. You stop every hour, take a pit stop, eat a snack and relax your contact points and that is about it.
You have a very good sign by the way. You like cycling! Go out and enjoy yourself. Just go out and have fun and most of the conditioning will come naturally.
I have seen people of the "no pain no gain" go out and grimly try to get better very fast. That is a big mistake. It is a good way to injure yourself. Plus, if every time you get on a bike it hurts and all the time afterawhile, you will probably stop. I am not saying not to go out and push things every now and then to see what you can do. A little pain can be fun. But to turn it into work and drudgery is a big mistake.