Sydney Road is a very long road - pretty much as long as you want it to be, really. I've just moved house across town, to somewhere near where it begins, and figured that "one day I oughta walk up to the end of the Sydney Road shops. You know, Bell Street or wherever it is." I'd mentioned this to the various people who helped me move, and one of them said "Nah. You oughta walk to the Ford factory, right on the edge of town - that'd be better."
I set off yesterday morning around 9, figuring I'd walk up to the end of the shops, but one by one the milestones kept coming and going - Bell Street, Pentridge Prison, the end of the tram line (at which point I figured I'd just keep walking and catch a train back, instead), the rather large Fawkner Crematorium / Memorial Park, the Western Ring Road, and upon reaching the intersection where I'd been telling myself "ok, this is it, I'm really going to stop when I reach those traffic lights" it was right there across the road.
Travel time was about 3 - 3.5 hours. My best advice ? Keep moving (at least, most of the time) - you'll feel worse if you stop to sit down. And wear comfortable underwear !