When we develop our brain, we should keep the carrying body healthy as well.
Being educated at PhD level and healthy at the same time seems to be crucial and really challenging nowadays.
Such future where brains are inside some kind of suspension and electrodes inside to get all the wisdom out of the brain would probably not what we would like to imagine. Let us develop the Healthy Scientist® brand.
So - the less you can sit the healthier you would be.
About walking when working
Speed should be slow - possibly 1-2 km/h but not more than 4 km/h - depends how precise work needs to be done.
standing desk: DuckDuckGo search | Google search
ergonomics of standing desk (image source)
treadmill desk: DuckDuckGo search | Google search
moving part can be e.g. veloergometer, stepper etc
http://www.nextdesks.com/store/fit - special softening cushions inside belt
http://www.lifespanfitness.com/workplace/treadmill-desks/tr5000-dt3-under-desk-treadmill - also Bluetooth connection with computer for statistics collecting
veloergometer without front part:
https://www.thefitdesk.com/ - small desk added
height adjustable table:
http://www.walktop.com/ - desk for using existing treadmill
desk and treadmill together:
http://www.lifespanfitness.com/workplace/treadmill-desks - one of the most famous
http://store.steelcase.com/products/treadmill-desks-walkstation - prices can be also so high...
http://www.workfittreadmill.com/ - 690 USD (~585 €) - the cheapest I've found so far...