Purab will give the money to Delhi-based NGO Friendicoes, an organisation that works towards animal welfare.
"I was approached by Friendicoes to run the marathon, a few days ago. They said, 'It would be nice to have your support'. But they had just one category left - 'Dream Team'. They said that I can be a part of it if I am able to collect Rs.100, 000 by Oct 28, " Purab told over phone from Mumbai.
With lady luck smiling on him, he has been able to collect Rs.45, 000 for the cause within 24 hours of sending an e-mail to his near and dear ones, he said.
The prize money for the second edition of the Airtel Delhi Half Marathon is $210, 000. It is expected to draw a galaxy of sports stars, Bollywood actors and celebrities from other walks of life who will join thousands of people in the marathon.
Purab is unsure if he will be able to continue his association with Friendicoes.
"Friendicoes is in Delhi, so it can get a bit tough to associate with it but I am still trying to learn a little more about them and the kind of work they do, " he added.
Till then, he is busy practising to run "at least 20-21 km" for the marathon.
"I was supposed to run at the Mumbai Marathon as well, but I fell ill. I am trying hard for this one. I ran about 20 km on Saturday. Usually I run twice a week for two hours, " said Purab who was seen in films like "My Brother Nikhil", "Woh Lamhe" and "Rock On!".
Right now he is waiting for the release of Apoorva Lakhia's "Hide & Seek", a murder mystery about six friends, which may release in December this year.
"People have always looked at me for a different league of cinema. But I like doing different roles and message-based cinema, " he said.