[limey]

MTP, please try to work on not feeling guilty for taking the breaks that are yours by right and using them to improve your health (either through eating right or or gently exercising). Maybe search the internet for yoga-type exercise you can do at your desk - this would not strain your knee or hip (or not so much) and would start to tone parts of you. Maybe keep the cooler-bag of snacks in your office under your desk (where it will be cooler through the day than in your car) so that you can dip in to something healthy on your drive home?
I'll confess to having a really strong objection to anyone sacrificing their own health and time (including breaks, holidays, and donating free overtime, not that I'm saying you're doing all of that) for their employer. IMHO no employer ever appreciates it, they simply take it for granted. Please try to take you breaks.