personally I think you can do a little bit to practice. Being able to swing your arms in the right order counts for a lot , i.e., not tick tocking, which seems to be the mk1 problem most people seem to have. Just remember, marching is simply a formalised version of walking. Start off walking naturally and then build in the rigidity of the arms and the measured pace.
Additionally, left/right/about turn/saluting can also be practiced, once you have been shown how. What you could do with is finding a drill instructor at a local cadet force and asking them if they could sharpen you up. I am pretty sure most would be pleased to help.
If anyone hasn't recognised your tenacity through all this, then they are missing a solid character trait! I hope it works out for you.