I used to have fairly regular sports massages and the therapist , who is also a qualified osteopath so knows her stuff, would make sure she did not go in too deep in any session a week before an event.
I have a fallen arch and wore orthotics successfully for a few years. Then forgot to wear them on a backpacking trip and damaged my posterior tibialis tendon as a result. Have since had heaps of physio focussing on 'listening foot' exercises which have enabled me to have a better foot position plus glute strengthening. So, I think one's foot position possibly can be improved and I am left wondering if I had taken this approach in the beginning rather than having orthotics maybe I would not have got injured? Who knows.
You do not need to be able to do a 5k in a certain time to do a 10k event. Just increase your long run once a week by no more than 10% until you are at 10k distance.
To speed up for a 5k, have you tried interval running? ie running a bit faster than you usually do for very short periods like 1 minute with a minute or two rest at a slower speed in between. Best to get more info on this and find something that works for you but this could easily be done on your treadmill runs.