go for it man, dont put it off, I hit 18 stone beecause I took 2 years out of muay thai due to injury, after I was better i put off going back because I wanted to get into some kind of shape first, it wasnt untill one of my friends form Northern Ireland said "you have an amazing Muay Thai/MMA gym on your doorstop if i lived in glasgow id be their every day!!!" and he was right so i went back and im now 15 stone.
Go for it and be realistic because it will take time and you will get the odd injury but just keep at it
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Its not about how hard you hit, its about how hard you can get hit and keep moving forward