Wilbur is great basic terrain height map editing tool to create ArmA 2 terrains.

1. Filter → Noise → Fractal Noise, ridged multifractal, operation: replace, H: 1, amplitude: 1, lacunarity: 1.9, displacement: -10, octaves: 7, XY scale: 2 and other box 2, offset: 1, XY origin: -10 and other box -10, fgain: 2, seed: click random button
2. Filter → Fill → Fill Basins (CTRL-B), use default -1 value
3. Filter → Mathematical → Span, Low = -60, High = 1000
4. Filter → Noise → Percentage Noise, 5% (5% is very very rough, suggest you try smaller values)
5. Filter → Fill → Fill Basins (CTRL-B), use default -1 value
6. Filter → Erosion → Incise Flow; blur 2, amount 3.5, flow exponent 0.35, effect blend 0.25
7. Filter → Fill → Fill Basins (CTRL-B), use default -1 value
8. Filter → Erosion → Incise Flow; blur 0.5, amount 2, flow exponent 0.1, effect blend 0.1

That makes quite nice looking eroded terrain.

Wilbur sports a fluvial erosion model in addition to the incise flow operation. Incise flow requires a connected drainage system for best effect. The fluvial erosion system works either way. Wilbur uses a fluvial erosion model based on the precipitin or waterbot models. It randomly drops an agent onto the surface and that agent travels downhill, moving altitude from the higher cell to the lowest one adjacent to the current one until it finds a low spot.

Filter → Erosion → Precipitation-Based, use values: delta: 0.25
max length: -1
connectivity: 8 way
wrap: none
passes: 3
blend: none
amount %: 50
noise: none
amount %: 1

Want more detail? Resample the surface. Surface → Resample → Simple