I too came back to playing after a 15 year layoff. There are no brass teachers in my area so I did the wrong thing. I bought an Arbans book and started at page 1 and tried to work my way through the book. What a mistake. After 10 very frustrating years with only sporadic and non sustainable progress I went to the internet for advice. This helped a great deal in correcting some well ingrained bad habits. I also realized that I need to find a good instructor, which I am in the process of doing. One bit of advice that I took off this forum is using slurs. I tended to stay away from doing them because I simply wasn't good at it. Now that I have done them for some time my progress is much better. I have talked to some returning players and hear very similar stories to yours and mine. Use of long tones and sluring seem to be a very good way to start on the road back.