Beside I don't know your final resolution and other details, you could try to start to tweak for some acceptable output with that (works 800xXXX - up to full HD):
Detail 0.5
AA 6
Defer atmo on
atmo samples 12 (atmo jitter 1)
No! costumized sampling for the beginning
Microvertex jitter off
Detail jitter off
Raytrace shadows on
Raytrace objects off (if no objects)
Anti-aliasing bloom off
Detail blending 0.5
Displacement filter 1
softclip effect off
contrast off
Ray detail region padding 0.5-1
sunlight 2-2.5 (you have to tweak the atmo haze and stuff settings))
Enviro light / strength in atmosphere off (0) safes rendertime, fake light that with the tweaks in atmophere tab
Ray detail in motion off (subdiv. settings)
Ray detail multiplier 0.7
fully adaptive on/off, your choice, test both (hit animation check button before testing both manually)
clouds quality 1, jitter 1
acceleration optimal
GI settings:
2/4/8
Supersample prepass on
GI prepass padding off
GISD 1/1/24
* I forgot: Catmull-Rom pixel filter
Good luck !