This solution will address the following achievements:
If you follow this solution, you should get most of the way to all of these achievements during an ordinary playthrough.
When you start the game, you will not have access to either bikes or the skateboard, so you will be running around 100% of the time. Later in the game you will unlock the skateboard, and once you open the gates of Bullworth and can visit the city, you'll have access to the bikes.
Some overall ideas to keep in mind:
* Bikes are only available at specific locations or by hijacking someone riding one.
* You carry the skateboard with you.
* You can run anywhere.
My suggestions:
* When in Bullworth Academy, always run. 100 km is a lot of distance, but you'll spend enough time running around there (and in spots outside of the Academy, during missions, etc.) to accumulate distance.
* When traveling to/from/in the city, always use a bike when it is available. There is almost always a student riding a bike outside of the Academy when you leave (typically approaching from the west or departing to the east) making that an ideal time to get a bike. There are bike stands throughout the city areas of Bullworth in case you need a new bike.
* There are a fair number of bike races and paper route missions; you'll add distance to your bike total on those as well.
* For all other times (outside of Bullworth and no bike available), use the skateboard.
* Once you get the bike achievement, use the skateboard exclusively outside of Bullworth Academy.
My guess is that if you do the above, you should have all of your bike and foot distance by the end of the game, with the skateboarding distance either complete or close to it.
If you have some skateboarding left to do, just leave Bullworth, find the first car you see, skate up behind it, and press Y to skitch (grab the bumper). The car will pull you around endlessly (until you pass out at 2am, of course), and it will count for skateboarding distance.