A solid-crust planet with a radius of 40,000 km? With an Oxygen-Hydrogen atmosphere? Oh my. Where do I start?
We have to start from the basics.
Planetary radius
It might not seem like a lot, in space. We are, after all, used to millions and billions of km in the solar system and light-years in the galaxy. However, when it comes to planets, tens of thousands is a lot. Let's give your planet a good name, like Blobby.
Planet Mean Radius (km) Mean Radius Compared to Earth (Earth = 1)
Mars 3389 0.52
Venus 6051 0.95
Earth 6371 1.00
Neptune 24622 3.86
Uranus 25362 3.98
Blobby 40000 6.27
Saturn 58232 9.14
Jupiter 69911 10.97
So Blobby is about bigger than the ice giants, and half-way between Neptune and Saturn. Ok, so it's like 6.3 Earth radii, no biggie, right? The reason why this is a big deal will become apparent as we move on to the next sections.
Planetary Volume
Using a Fermi approx. and assuming it's a perfect sphere (and leaving aside the atmospheric weight), we can use $\frac4 3\pi r^3$. For Earth, with a radius of 6.371 million meters, that's roughly $1.09\times10^{21} m^3$. Your planet's radius is 6.27 times higher. 6.27 cubed is 247. So your world would have about 250 times the volume of the Earth. Are we getting worried yet?
Planetary Density
The density of the materials comprising a "rocky" planet can vary roughly from $3 g/cm^3$ (pure rock) to about 8 $g/cm^3$ (pure metal). In addition to this, the force of gravity compresses the planet somewhat, making the radius smaller and the density higher. The bigger the planet the higher the mass, obviously, so Earth (5.5 $g/cm^3$) is more compressed than the Moon, Mercury or Mars. Blobby is big. Going forward, we'll assume Earth density (given compression, it'll be about 70% rock, 30% metal). Just for fun. In reality, there is a trend where planets with radii up to 1.5 Earth radii increase in density with increasing radius, but above 1.5 Earth radii the average planet density rapidly decreases with increasing radius, indicating that these planets have a large fraction of volatiles by volume overlying a rocky core (in other words, more like Neptune than Earth).
Mass
If we somehow assume Earth density of 5510 kg / m3, given Blobby's volume, of $2.68\times10^{23}m^3$, we get $1.47\times10^{27}$, which is rather close to the mass of Jupiter (with 2/3 of the radius).
Surface gravity
You can estimate surface gravity by using this formula:
$g=\frac{GM}{r^2}$
Our surface gravitational acceleration is 61.6 $m/s^2$, that's 6g. That's crushing. Not only do you not get 20,000m tall mountains, you're lucky to get little hills. Even if we reduce planetary density in half, you still get 3g. If we let a lot of the radius be gas and clouds, you get crushed by the atmospheric pressure instead. There's no way out.
So that covers the solid crust planet with the ginormous radius and the outsized mountains.
Let's move on to the part where the atmosphere is 20% oxygen when there's lots of hydrogen (30%). No, no, no. Oxygen is a reducing gas. The atmosphere would ignite given the first spark (say caused by lightning). Pure hydrogen-oxygen flames are so energetic they emit light in the UV spectrum. Your whole atmosphere would go down like the Hindenburg.
TL;DR: Hell, no. That sort of whale of a planet would either crush you to smithereens though gravity, or squish you flat as a pancake by sheer atmospheric pressure.
