It's looking good.

Here's some advice from my similar experience:
- starting from bottom level to top level will often lead to higher altitudes, it's a good idea to decide, from the start, what's the highest you want to go;
- sometimes, it's better to start from the top;
- I assume you are doing each level in a different layer, if not, you're doing it wrong and FIX IT immediately;
- try to do it in a way that altitude profile isn't the same on the two sides of a mountain range - that's rare, more often than not, one side is steep and the other is some kind of plateau);
- don't forget that some areas will be flat and low, where large rivers drop the sediment - have a look at the topographic maps of the amazon basin or the ganges mouth or the orinoco...

keep it up!