Ok, I'm starting to be a little confused....
Usually I prepare my FP in the FMC inserting Departure, Arrival, the route part, the Perf Init data, flaps, cg etc. until Preflight Complete is obtained.
Then, a soon as ATC assigns it to me, I insert the SID.
At this point, all work like a charm, once in flight on the CDU Progress page I can read the distance to TOC, but, once in Cruise phase, I have no distance from TOD shown, unless I insert a runway or a procedure (STAR or APP) for the Arrival Airport.
I was thinking, as "piston broke again" said, that the FMC was able to compute a raw TOD knowing the arrival airport elevation, and then had refine the calculation once others data have been inserted (procedures, winds, runway etc.), but it seems not so confirmed so far.
Anyway, thanks to all the guys partecipating at this discussion, for me is really interesting
