You could keep doing the work you're doing, but without the sitting: get a stand up desk (if money is an issue, you can get a cheap hand-cranked desk from Ikea)

You could keep doing the work you're doing, but without the sitting: get a stand up desk (if money is an issue, you can get a cheap hand-cranked desk from Ikea) and get an under desk treadmill (harder to avoid spending at least $1k, sadly). I get in about 5 hours of walking per day this way, at 4 mph (I'm 6'2", so that's right around my typical walking speed). I feel infinitely better this way, and it's also an incredible way to burn off calories with minimal intensity.