200 kilometers is a pro/elite distance for races. Is it a race or organized ride???
Regardless, you won't be able to do much with 3 weeks to train. If you are well conditioned, your body can handle and recover from a lot of intensity. If you are starting out, you are more likely to get injured or overtrained. Be sure you can handle the kilometers!!!
Everyone who loves cycling must be aware that it takes many years of regular, hard training, to get to a high level, regardless of your goals. Think of your training in terms of years not weeks. The joy is in the journey. Once you get to the top, what is left to accomplish???
If you are going to do the ride, using your old bike is the smart choice. You are used to it and has the necessary gears for climbing. Use you old bike!!!
Good luck!!!