Some good stuff along the shoreline between Granton and South Queensferry, including a wee pump track at Gypsie’s Brae (Cyclone Park) and some nice (if flat) muddy/rootsy singletrack heading west in the trees.
Once you’ve crossed Cramond Brig and descended on the NCN route to the shore of the Forth, there’s some great singletrack (again, quite flat) that leads most of the way to Queensferry.
I use a lot of this as a CX training loop to/from work, gives you an idea of what to expect.
Once at Queensferry you could head uphill to Craigie’s Deli for cakes and trails in the forest there.
I’d suggest most of the above as a ‘scenic diversion’ rather than a solid MTB epic, but should keep you amused if you’re not too scared of pedalling 8)
Edit: attempt at mapping some of the above on google maps. Obviously doesn’t show all trails, between Gypsie’s Brae and Cramond the good stuff is up in the trees.
https://goo.gl/maps/TQvhEL9wGVU2
Could show you around some evening after work, often take this route home anyway.