IMO strength is good to have when you want to lift something or move it,but as far as aikido goes. You dont realy need strength. I am a realitivly big guy to and at first i had a problem with using technique and not strength. you have to put your strength away. If you are using strength in Aikido your doing it wrong. O'Sensei was asked once, " Are you the same man with the same power and strength as you once were?" His answer " I am still that same man with the same power, as for strength goes, as you get older eventualy you will loose your strength". This is why when you are young and healthy you must get away from using your strength pratice technique. When you get oler you wont have the stength to fall back on but technique will always be there.