Philippa suggests you to really analyse how you sit to sew. Take a good look at yourself and even get someone to take a photo for you to look at. Don't alter what you normally do when this photo is taken.
Is your machine too high? Probably yes if it is sat on a table. Much better to have it at the same level as the table. Is it too far away from you or too close to the front of the table? Philippa's machine measures about 11" from the needle point to the front of the table, and is 27" from the floor. Sewezi (both versions) are brilliant at correcting these issues and you can buy risers for the legs if you are very tall.
Chair height and position - is this good? A padded, height adjustable, no arms, swivel chair should cover all options. Get your chair to the right height and not too far in/out. Be sure to be sitting square on with your needle in line with your nose. Even sitting slightly off to the side will result in a skewed spine and aches later in the day.
Where is your foot pedal positioned? If it is even slight off to the side/too far out you will be twisting/straining to reach it. It should be in line with your hips and comfortably beneath your foot. Make sure there is no junk on the floor to stop you getting into a good position.
This is all a big ask for your body. You have not trained it for sitting hours hunched over a machine. If you were a non gardener you would not go out and dig for 10 hours solid and expect not to ache! You need to move often and to stretch to counter balance the contraction of your chest/biceps. Get up at regular intervals and go for a short brisk walk. Stretch often with shoulder rolls, neck gently side to side, hands clasped behind your back or stand in a doorway with your hands holding onto each side of the frame (fingertips to the back of you) and gently lean forward. You will feel a lovely stretch in your pecs and biceps. Ideally we should all be doing light weight training of our bodies to help with all of this (and more in our daily lives) but esp upper body and core. Small pair of light dumbbells for reps of shoulder presses, shrugs, lateral raises and similar - not forgetting abs and backs - which can be as simple as standing on one leg (holding on to a chair if you need to) and then the other, or holding your tummy in tight for a few seconds pretty often in the day. There is no stage in life when this is not beneficial. OK so hope that this advice helps. You did ask (and yes, Philippa was an exercise instructor for a lot of years too!).