How long before I can do a push up? I have 375cc silicone gel under muscle.

In my practice, I tell my breast augmentation patients that they are able to perform physical activities without any restrictions (eg. push up, weights, etc)  AFTER 8 weeks post surgery. Before that, the implant can shift, and asymmetry may occur. 

You should be able to exercise fully at this point (3 months postop) so just work your way back up to what you were able to do prior to surgery. Obviously, since you haven't worked out in awhile, you will need to build yourself back up to your prior routine. Good luck!