Old Mutual Two Oceans and Comrades Marathons have closed entries eight weeks before the deadline due to high demand and meeting their targets early.
The Comrades, with Old Mutual as one of its major sponsors, has an entry cap of 20 000 while the Two Oceans ultra-marathon accommodates 11 000 participants.
Organisers for both said long-distance running was thriving in South Africa and across the world, with athletes from more than 70 countries among the Comrades entrants.
Entries for the Old Mutual Two Oceans Marathon closed in record time when the event reached its limit in the early hours of 02 January.
This was eight weeks before the scheduled closing date and 83 days ahead of the event itself.
Entries for the Ultra Marathon sold out a week before the official closing date for the first time in 2014 and one month early in 2015.
Runners who failed to secure an entry can try again when the substitution entry window opens from 11 January to 23 February.
The 2016 Comrades Marathon will be the 91st running of the event and will be a down-run from Pietermaritzburg to Durban this year. It takes place on 29 May.