Well i play inside center and have done for many years, the thing about center is you need to be solid in defense and attack. The fact that you can tackle is great but you also need to be able to create space and set up your outside man. As far as speed goes if your not so slow that you will struggle to get back in position after a phase or if you cant keep up with your players in attack it wont be a problem if your not super fast. If you're not that fast i would advise that if you want to play center that you play inside as that is more of a play making position as apposed to a try scoring and 'fast running' position. If you find you arnt suited for center I would advice that you try and play flanker. As flankers are on the outside of the scrum you will find your self a lot in the backs, also if you are a good tackler flanker can be a good position
Forwards who tackle in the loose think it's easy ... when the ball goes wide it's completely different as you have to chase and tackle from behind rather than just get in someone's way
Get fit, lose the podge, and work, work, work on the speed. It's good you can tackle, but it is only useful if you can catch the sub 11 second for 100 metre winger.
