They freely walked on this planet because you are correct in that the planet's own gravity wasn't very high, not so much higher than earth's gravity. Iin fact they explicitly say in the movie that the gravity is 130% of earth's gravity.
However, the extreme relativistic time dilation they experience (1 hour on the planet equals 7 years on earth) comes not from the planet but from the black hole Gargantua's gravitational influence, which has an extremely high mass (approximately 100 million times the mass of our sun, according to scientific advisor Kip Thorne) and to which the planet is very very near. This is what causes the huge time distortion relative to the station (which is in a farther orbit around Gargantua and thus isn't subject to its heavy time dilating effects).
The reason they in turn don't physically feel Gargantua's very high gravity -- in the sense of getting abnormally dragged towards the planet or the black hole -- is because the planet itself is on a stable planetary orbit around Gargantua and thus in a state of continuous freefall (in the same way we on earth don't actually feel the sun's high gravity but only earth's due to orbiting around the sun). But the planet and its inhabitants are still subject to Gargantua's extreme gravitational influence and the resulting time dilation, of course.