I'd think it's at least 2 miles from Taplow or 2.5 from Burnham to get to the lake entrance, and possibly a little more. Plus every event I've been to starts near the Boathouse, so that's another 1.5 miles or so on top to get to that end of the lake. Taplow Station is the closest, but the road is narrow with no footpaths. An alternative would be to go to Windsor & Eton Riverside from Waterloo then walk along the Thames Path to Dorney Lake. It's further (and a longer train ride), but much more pleasant and traffic free. Bring a bike perhaps.
Edited: 04/03/2008 at 11:15