How did you go faster than s-l-o-w?

I'm new to US Masters Swimming and am a fitness swimmer trying to get faster and build endurance. Anyone who's ever swam (swum?) in a meet should stop reading right now. Because my fastest 50 is about 53 seconds and it takes me about an hour to swim an 1,800. I'm 53 years old, working hard on technique (I have a friend who's a swim coach, and am using Emmett Hine's book "Fitness Swimming", plus a training snorkel at times. I can't wear fins because my feet cramp.). I do some 50s on 1:15 with the pace clock almost every time I swim now and seem to be a little faster. Learning a lot and it feels great. Will begin strength-training this week, too. So, do you have any advice for a fitness swimmer--what helps with speed? thanks!!! Sari