If I am understanding you correctly I am not aware of any way to do this except having the key frames properly (evenly) spaced from the outset:
- If your first movement is 10 meters and 10 seconds long and Second movement is 10 meters and 20 second long: just move the key frame from 20 seconds to 10 seconds total so that they both go 10 meters in 10 seconds
- If your First movement 10m - 15 sec and Second movement 10m - 15 sec you would key frame the first at 15 seconds and the second at 30 seconds, again so they go the same distance in the same amount of time
You will have to do this from the start; You cannot adjust an existing timeline because going longer distances in the same amount of time will always result in an increase in speed.