Not a bad start, Daveash. Some suggestions:
For muddy ground, nothing works better than actual mud. This is the way that I do it:
1) Carve out the general shapes into the base out of whatever material you happen to be using, and sand till smooth and natural looking.
2)Paint the base brown
3)Make a mud mixture out of fine earth, and a 50/50 mix of water and white school glue (you can experiment with this ratio, and see what works best for you)
4) Take this mixture and spread it out onto your base. While still wet, press any track marks, etc into it. You can also apply this mixture to your model to tie it in with the base.
When dry, it will form a hard surface that can either be painted or left as is, as it will look like actual muddy earth, because that's what it is. To represent really wet areas, spread some gloss coat or even more white glue to give it the 'wet' look.