How long does it typically take to train a MNIST data on a Mac Pro?
Did you try to do a model summary and check the size of your model. Yes, i may take hours as you are adding lot of layers. Try starting with 10 or 20 epochs and check the accuracy of your model gradually. May reduce your training time.

When you do the above exercise its better to have a validation data set, rather than fitting the model on the training data set time and again.