Hi, I've got some details on portions of the Lerderderg Track on my website -
http://bancoora.com but these are in a number of separate trails under the Blackwood and Lerderderg sections of the site (eg Byers Back Track, Lerderderg Heritage Walk, Blackwood Ranges Track, etc.)
The GDT (of which the Lerderderg Track is part of) is open to MTBs and walkers and the GDT website only excludes the section within the conservation zone of the Lerderderg Park (Whisky Track) from MTBers. Recently the GDT had an article in the Blackwood News indicating that the Lerderderg Track may be discontinued from being supported due to the trail getting overgrown and not supported so I've been clearing the 10 kms out to the west of Blackwood and in the last few weeks someone else has had a good go at cleaning the trail up as well so I think things may be getting better. For this reason, the GDT had ceased selling the map for the track but you can buy the Wombat State Forest Touring Guide map from Meridian Maps and this still has the trail marked on it in purple from Daylesford right down to Bacchus Marsh. It's a good map and you can buy it here.
http://www.meridianmaps.com.au/products_images/product_links/4wd_maps/wombat_tour_guide.asp. If you want to do the ride, I'd follow the track as per the map but bypass the Lerderderg State Park bit by using the Telstra Track on the Bancoora website, then Mount Blackwood Road then pick up the track over near Mount Blackwood - the run down into Bacchus Marsh is great fun.
If you're interested in doing it, drop me a line at the Bancoora email address and I'll give you a bit of guidance around the Blackwood area as there are some better alternatives to that shown on the map. For a fit rider it's probably a two day trip and you could probably spend a day in Blackwood as well checking out some of the trails.