Training CNN for Regression
Although building neural network models is admittedly still an art rather than a science, there are some (unwritten) rules, at least for initial approaches to a problem, such as yours here (I guess).

One of them is that dense layers with 50,000 nodes are too large, and AFAIK I have never seen such large layers in practice; multiply this x3 (layers), and no wonder your code takes forever.

I would certainly suggest to experiment with a dense layer size between 100 - 1000, and even start with less than 3 dense layers. Reducing your 1st CNN layer to 32 is also certainly an option.