This video features an intern for the Edmonton Capitals (Golden League) a few years ago who attempted to tryout with the team. While this was uploaded likely as a joke or spoof, you would be surprised how many players attend independent baseball tryouts with barely little more muscle than what you see in this video:
While speed is a great gift to have, as a position player you are going to need some muscle to withstand the rigors of anywhere from an 80-game to 100+ game schedule; and often you will be playing in weather conditions far different from what you are used to experiencing growing up. Check out the resources page for one of the better programs to put on muscle before you attend any pro baseball tryout.
Become aware of upcoming tryouts by joining the notification service here: Independent Minor League Pro Baseball Tryouts
If you need some help increasing your 60 yard dash speed times, arm velocity, vertical leap ability, hitting power or improve your muscle mass for any professional baseball tryout (or private workout) then here are some resources to help you: Resources for improving your chances at any pro baseball tryouts