Hi tilemanby,
there are mainly 2 kinds of cardiomegaly:
1- ventricular hypertrophy;
2- Dilatation of the ventricle.
Thus, the evolution of your condition will first depends of what kind of cardiomegaly we are talking about.
In each case, a lot of things may cause the cardiomegaly. So treating the pathology should first consists to clearly identify the exact cause that induced the said pathology. High blood pressure is often associated with cardiomegaly. And it may be the cause. But it can also hides another cause, that one inducing the high blood pressure (eg. 1- main cause X - 2- high blood pressure - 3 - cardiomegaly).
In other words, you need to know:
1) the exact kind of cardiomegaly;
2) the main cause of the pathology.
This investigation should be done by a cardiologist. So once it's done, the cardiologist will be in a better position to tell you exactly what are the activities you can practice or you can not.
Depending of your exact heart condition, some exercices may worsen the situation. But generally, walking is the better exercice you can do and it rarely induces bad consequences. It will/could increases your ventricular function maybe helps your blood circulation.
The main point is you should be treated by a cardiologist