The man who set a world record of 8,891 points in 1992 still looks fit, but ruled out any possibility of running on Sunday.
"I cannot run now. I look fit because I regularly work out. That's how it has been throughout my life," said the Olympic gold medalist (1996 Atlanta Games) and three-time World champion (1991, 93 and 95) in a chat with the media in Mumbai.
O'Brien is honoured to be part of the SCMM this year.
"My role is to promote the sport in India. The Standard Chartered Mumbai Marathon has put a great effort in creating a running culture here," said the 44-year-old American.
He is also the oldest athlete to win an Olympic medal -- at the age of 30. He, however, felt he could have won it earlier on in his career had a miscalculation not occurred in 1992.
At the US Olympic trials, O'Brien voluntarily passed on the lower heights in the pole vault, then failed in his first three attempts, resulting in no points for the event; ultimately, he did not qualify for the 1992 Olympic team.
"It was a huge lesson for me. I required extra-ordinary effort and motivation to realise my dream," he recalled.
He credited his coaches for all he's achieved.
"Decathlon is a technical event, and without them I would not have been able to be what I am," he said, reflecting on the ten track and field event discipline.
"I hated having too many rest days. Marathon is similar to decathlon when one completes it. It is a great feeling of accomplishment."
About decathletes not getting enough due for their efforts, he said: "To be a decathlon athlete one needs to be humble. We don't make the noises, but I guess the media does recognises our efforts.
"I may not have become as famous as 100m or 200m champions, but I also had my days in the limelight."
Asked to highlight the changes that the track and field has gone through in the last decade, he replied: "I don't think it has changed much. In fact, it has become much more professional now. Budding athletes are getting good money, which is so crucial. There are many more people coming to watch the T&F at the stadiums."
He also credited Usain Bolt's sensational record-breaking feats, saying it is a tremendous boost for the sport.
"Usain is a huge thing to happen to athletics. He has got the world's attention. Suddenly people want to know about this guy and what he is doing.
"Bolt has done what Tiger Woods has done to golf," he said.
Lastly, O'Brien was happy that not many athletes in track and field are testing positive for doping.
"I am all for stepping up the testing procedures if required. Dope testing is a part of any sport, but that's the price one pays to be a part of it. I would like to get the sport cleaned up 100 per cent. I did it in a clean way, which was one of my priorities in my career," he concluded.
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