I think it is best modeled by traits. I was fairly athletic. In HS I ran a sub 2 min half-mile and was consistently one of the fastest players on the soccer field. However, my max speed topped out at 12-15 mph. Usain bolt hits 22+ mph. That is huge difference and it is not even taking into account the "common" person. I just don't think there is any simple way to simulate that without specialized traits.
Enter the Athletics skill (and preferably a decent/high Strength score...).
That is the difference. Not everyone is trained in Athletics, or has
Mobile to gain +10 speed, etc. Most sprinters (especially at the college level and higher)
are fairly strong and would have a good or better Strength score as well.
And even the "common" person can "run" (sprinting is a different issue IMO), even if not for long.
The simplest solution (for a starting point anyway) is to have "Dash" add double your speed to your movement for the turn, effectively making a speed 30 creature able to move 90 feet. Honestly, even that is a bit slow but something most "common" people can probably do. That would translate into about a 22-second 100-meter run. It is sort of fast, but not really compared to anyone who can actually "sprint".
Next, allow skills to grant variable boosts like they used to. For instance, a DC 10 Strength (Athletics) check might increase your Dash multiplier from x2 to x3, allowing you to move 120 feet (remember to add the original 30, which I included). DC 15 would be x4, for a total of 150, DC 20 is x5 for 180, and DC 25 is x6 for 210 total. At 210, you
are basically Usain Bolt. Without Athletics proficiency and a decent Strength score, you aren't likely to make those higher DCs. If they seem too low, bump them all up by 5 even.
Add in a condition or two, like difficult terrain imposes disadvantage on the check, or perhaps a
very smooth area would grant advantage instead, and you have a feasible system.
For PCs, with feats and features, they
could perform better than real-life, but for a fantasy game I think most people would expect that at some point.
@dave2008, FWIW I ran in high school and college as well, but my longest event was the 1/4-mile (which frankly, I hated doing when I had to!).
