It is extremly unwise to sit on flights for long periods as , as you get older then blood clots can form especially in your legs. So much has been written about this.
I have for many years been taking flights about twice a week of duration more than one hour. The advice I have and follow is to get up an exercise , I walk the length of the plane every aopprox half hour so do all the regular fliers with me.
The only timer this is interrupted is during periods of storm and we are all greatfull to be held for that generally short period in or seat restraints.
Go fot it, you are only on this earth for a minute time period and all should make the most of it !!!! Dont miss any oportunity.