GOOD MORNING to you! :) I'm a big stretcher too - especially in the morning and often throughout the day.
When sewing I have my ironing board and cutting area away from my sewing machine, so I'm hardly sitting for long lengths of time.
I use a clock radio that has the 1 HOUR sleep timer on it. I press it - sew during that hour - and when the radio shuts off I do something else for a few minutes... flip a load of laundry, run upstairs for food or drink, walk the tread mill for a few minutes...you get the idea.
I agree - use it or lose it!