I just bought a marrow bone online and it arrived today - it's huge (like the size-length of my lower leg). It looks like something you would just give to a dog to chew on... not the small cut up pieces of circular bone with marrow inside I was imagining. I am kind of at a loss of what to do from here.
i would hack saw the joint knuckles, most collagen so this should go to a stock pot, then i would cut it about 3 inch pieces and roast them in the oven, i do roast all the bones that go to a stock first. Use a coarse saw, the marrow fat blocks the saw blade very quickly if its too fine (been there;)
I'm not sure where you live.. but are you friendly with any local butchers? If not, find one and just go in and ask if they'll bandsaw it for you into a couple of 5" pieces. Believe it or not it's actually more difficult to get a marrow bone cut in half, like a boat, as it's dangerous for the butcher. If you can't find one then grab a saw, lady! :) Take the rounds, oven temp to 450, on a foil lined baking sheet, or a pie pan lined in foil, standing up. It should only take about 15 minutes but as soon as you see the marrow start to separate from the bone, take it out. You don't want all that tastiness to leak out. Make a nice herb salad dressed with lemon vinaigrette. Now: spoon, scoop marrow, top with a little salt - Kosher or Sea, a little herb salad, eat up! Ok. Now I'm hungry :) Good luck!
Defrost it. (Put it in the fridge for a day or so if frozen.)
Preheat oven from 350 - 400 degrees F. Probably 400 because of the size ..
Place bone in roasting pan.
Put it in the oven and let the fun begin! Cook 40 + minutes (from the sounds of it ... closer to an hour for you ...). It is 'done' when all of the marrow is soft to the humble edge of a spoon awaiting deliciousness*.
*Note: sometimes the top cooks well and you think you are good to go eating it and then you get to the center and are bummed because the center and onward are still hard. :( Just stick it back in the oven if this happens ... This might happen to me ... because I'm so eager to eat it though ... ;)
EDIT: ... I feel I didn't answer the question well enough ... sorry ...