Hi, Scope have a factsheet about Ageing on their website. However, it would be difficult to predict what you can expect because cerebral palsy is a very individual condition and how it effects you will be different to others so no-one can tell you with any certainty how getting older will affect your condition.
Well done on running your first half marathon, hope you enjoyed it. Whatever your condition, it is important to keep fit especially as you get older. Stretching and swimming may help and I would suggest, if you are running and taking part in competitive sports that you get regular massages and/or physiotherapy to ensure you do not overstretch or damage your muscles. If you can find a good sports massage therapist, I think that would help maintain your fitness. A torn calf muscle is extremely painful so I expect you want to try to avoid doing that again. =.