Can't get much tamer than Sarn Helen between the Mountain Centre at Libanus
and the tarmac road which runs to Ystradfellte.  (Sarn Helen continues south
west of this tarmac road but gets a bit less tame...)

This part of the route is marked on OS 1:50,000 maps- most people would
probably start at Forest Lodge; the only tricky bit is a steep up-hill
section after crossing an attractive old bridge; this is lumpy and rocky.
If your 'old girl' is a free-lander you will not have the ground clearance
for it, but any series, disco or defender should manage fine.

Obviously all the usual principles of green laning and the green lane code
apply.

Cheers, have fun,

M.