This place should be your first stop -->
http://boxinggyms.com/
Second, if you're a linebacker, I'd imagine you're somewhat athletic, so you should be fine in that department. And don't worry about age, champions have started older than you.
Don't worry about getting any equipment at first, other than a mouthpiece and hand wraps. It's best to do most all of your training at the gym under a watchful eye, you don't want to develop bad habits.
If you think your football workouts were hard, wait until you get in the ring for a few rounds.
Good luck with boxing though, I wish you the best.
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