Forgot to add, make sure the colts feet are trimmed. Not cracked or broken off due to the sharp turns on the quarters it could break the hoof if its not been trimmed up which can happen with any horse of any age, Im sure you already knew that thou. I do 10 min sessions 4X a day for the first 3 days then move up to 15mins 4X a day for 5 days 20mins 4X a day for 8 days then final step is 30 mins 3X a day for the remainder of training. Each colt being different, adjust the session length and time taken to adjust enough to move up to your colt. Only other thing I can think of right now is TROT TROT TROT! Dont lope him/her hardly at all, trotting is what builds the muscle the most.
"A horse can lend its rider the speed and strength he or she lacks, but the rider who is wise remembers it is no more than a loan." ~Pam Brown