just dig. i find half the fun is digging and changing it to work better/expand/add a new line.
dont aim for something pro looking unless you have a mini digger as it is a lot of digging. i spray mine with weedkiller during hte summer as i dont use it enough to keep the foliage down on my own!
wouldnt bother to import – create borrow pits inside berms etc.
your garden is huge. my track is about 1/3 of that and it took me about 6 months of evenings to get a continuous loop that worked well. my rideable line is about a foot wide but everything is smoothed out and i`ve since added a few extra lines to cross back over and swap directions etc.
I`m toying with building wooden quarters….
whatever you do DONT buy a plan. orginality is best and most fun.