Running is very hard on the body. It's a high impact activity. With every stride you are putting the force of several times your body weight onto one leg. The impact is absorbed and dissipated throughout your body, especially in the leg joints and then up into your back, etc. When you start a run your body is fresh and rested and better able to deal with the stresses. As you run the cumulative stresses build up and things start to tire and hurt. We each differ in how much pounding it takes to reach this point. Beyond that it's mental toughness that allows some of us to endure the pain and continue on where others would stop.

The body is amazingly resilient, adaptable, and self healing. Increasing your workout in small increments allows the body time to adapt to the stress through increased bone density, and increased blood flow to muscles, tendons, and ligaments and to repair the previous damage.

Contrast running to riding and you can see that in riding there is basically no pounding, just the vibrations that are transmitted through the tire and frame to your hands and bottom. The stresses on the body are entirely different.

Personally, I find 2.5 miles of running to be far harder than 25 miles of biking.