Check here:
http://www.msblnational.com/main.aspx
I haven't played in 3-4 years, but when I did it was fun. Big time sink. 9 inning games. Some leagues are wood bat only which is fun.
The same positions that the pros scout for are the same guys needed in senior ball: 1, 2, 6, 8 and maybe 3 if you're a lefty.
Pitching is a mixed bag in 30+, fastest will be mid-80s. A lot of junk.
Make sure you bring water, ibuprophen, sunscreen, eye black. Wear a cup and get some of those new compression clothes, particularly for your hamstrings.
Otherwise, if you can play, they will find a home for you.