I don't train in Bel Air much, but I do know Annie's playground is a great spot. Huuuuge wooden castle with shredded tires instead of mulch.
As for whitemarsh,
-Perry Hall High, Middle, and Elem have some decent stuff if you know where to look. They are all within a mile of each other as well. . .
-Behind the bowling alley and rite aid there are a lot of nice rails, but it doesn't really open up until dark. -Behind goodwill and wendys is one of my favorite spots ever.
-St Joes church/school.
-if you walk far enough down belair road into rosedale there is so much stuff but most of it is private so I would stay away from it.
-Also in rosedale is golden ring middle/park which are both amazing. Overlea High and library are pretty sick too. The library is well lit at night, too.
Thats where I train mostly. I'm always looking for new places so let me know if you find anything