Beta Blockers interfere with the sympathetic nervous system. At the ends of the nerves going to the heart they release a neurotransmitter
called Ach. This then binds to your beta receptors in the heart and increased the strength and rate of contractions. As the name suggests beta blockers block the beta receptors in the heart limiting the sympathetic effect on the heart, which helps to prevent arrhythmias from developing.
In taking beta blockers, your ensure your parasympathetic nervous system
dominates, so the strength of contraction of your heart is reduced as well as the rate.
So beta blockers do weaken your heart but only when you take them, it is not a pertinent state. However there are other mechanisms in your body which will also intervene. If you started to exercise you will be fine (as long as it's not too vigorous to start with). As you start to exercise, your heart rate
will increase as the O2 in your blood decreases (due to muscle contraction
). This increase in heart rate means more blood will return to your heart per minute so the volume of blood in your heart will increase (increased preload). If you increase the preload of the heart you will increase the contractility of the heart muscle. ( because the muscle will be stretched more).
So by doing exercise you will strengthen your heart muscle. I would ask a physiotherapist or doctor to suggest an exercise plan to get you back to your active self, you will probbably feel better for it! The beta blockers are acting to protect your heart not damage it.