Kylie,
The assumption of linearity across runs might be overly restrictive. I recommend treating Run as a categorical factor instead. To implement this, you could modify the model specification from:
-model "Group*Run+(1|Subj)" \
to:
-model "Group*Run+(1|Subj)+(1|Subj:Group)+(1|Subj:Run)" \
For additional guidance on model specification in 3dLMEr, you may find this blog post helpful.
If you treat Run as a categorical variable, the lines specifying those post hoc tests via -gltCode will need to be adjusted accordingly.
Gang Chen