The NT cycle routes are not all that bad. I have done the main ones many times. There are many terrible riders, but if you are a fairly competent commuter you can avoid them and still maintain decent speeds for much of the way. If you are riding with someone who is not a competent rider (i.e. is not comfortable weaving through obstacles) then the paths might be more frustrating and slower.
If you want training mileage, Lantau is a decent option. Rent bikes from Friendly (Mui Wo) or Sunny (near Tung Chung but hard to find the first time) and do the tough Buddha climb. Bugger all traffic, but very steep. The stores mainly do mountain bikes though, not ideally suited to mileage.
People might move out of the way if you shout at them, but that won't make for an enjoyable day (and will probably just cause people to crash). The two stores I mention can provide helmets.
Do rental bikes come with bells and does anyone follow any system of moving out of the way? (Like Germans do....although I seriously doubt any other culture in the world takes these cycle or ski lane "rules" as seriously as Germans do.) Also, is it possible to rent a helmet too?