Thanks for the suggestions. Alas I had left before getting a chance to read too many of them.
I ended up riding from Alexandra to Eildon. The road had a good shoulder in places and considerate vehicles in all the others. Eildon's small, so I rode past the township turnoff (without really realising) and kept going to the Boat Club, which was a nice little hill and fantastic views.
Back down the hill I turned left to explore the dam wall and see what was beyond. Turns out the road continues around and heads back into Eildon - so you basically do a loop.
Was meant to be doing a TT effort, so after a think (no hills to TT up, wtf do I do now??) I decided to do another lap, TT-paced.
Did the short climb to the dam entrance (~7%) much slower than I had hoped, and when a car came up behind me, used it as motivation to push myself. Didn't go all that quick, but was pretty happy to see the HR rise to around 180bpm and then sit there for a bit (180-182 bpm for 1:25, HR(max) is 184bpm)... I think sleeping in that morning until 11:00am must have been great for the recovery.
Roads were wet and I felt uncomfortable really bombing the descent on the other side but managed to keep the HR above LT (170+bpm) for most of the circuit. Finished just past the bridge into Eildon, feeling like I had done what was required.
Ate more food over the weekend than I can recall eating in any 2-day stint previously and managed to put on a bit of weight but the training goes nuts for another three weeks now, so that shouldn't be a problem.
The valley is green, there was mucho water in the Lake (??) but the dam looked decidedly devoid of water. Impressive structure all the same.
Would be quite happy to return for some more riding, in possibly warmer / drier weather. It looks like there are lots of little side roads and hilly bits and pieces to go and explore too - Jamieson turnoff looked tempting.