Just kidding. But actually, it's true.
It is true that your mass (and your weight along with it) increases when you run. The reason people don't notice it is that it increases so very little.
As with most relativistic effects, they are most noticeable at speeds close to the speed of light. The muons (see "length") that are produced 10 miles up by cosmic rays shoot off at speeds of about 99% of the speed of light. The mass of these muons increases to about 7 times their "rest mass." That means they are 7 times more sluggish, and gravity acts on them with 7 times as much force, as when they are moving slowly (as they do in the lab).
If you travelled that fast, someone standing still and trying to push you to go faster would find that, if your normal weight is 140 pounds, you now weigh a thousand pounds.
You can see from the numbers above for the muons and the protons, that as the speed of objects comes very close to the speed of light, it becomes increasingly difficult to give them any extra speed.
In the book you will see why this produces a cutoff at the speed of light, 186,000 miles/second. Nothing can go faster than that. Not for reasons of technology, but they become so sluggish that it becomes impossible to force them past that speed barrier.
Onward to Chap 11: Mass-Energy Equivalence
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