What is the best way to condition and protect a hardwood floor under heavy use?
If the rolling of office chairs is your major concern, consider putting urethane wheels or casters on your chairs.  This type of wheel will not scratch or mar the floors further. These can be found at office furniture retailers. There is no way other than using chair mats, carpet etc. that I know of to protect a wood floor if you do not want to apply additional finish coats to entire floor.