Not only did England's record try-scorer complete the 1.5km swim, 40km bike and 10km run challenge in a time of 3 hours 10 minutes - almost an hour ahead of his initial target - but he is also benefiting Help For Heroes, a charity close to the heart of the former RAF pilot...
"I enjoyed it, I had done enough training and, although I was just aiming to complete it rather than 'race' it, I am really pleased with the time," said Underwood, now a Tigers director.
"First of all I wanted to get under four hours, then I hoped to break three-and-a-half, so I was pleased with that.
"A friend of mine who completed the Iron Man Triathlon in Zurich and is taking his coaching badges has been helping with my training and it was a completely new experience for me but now I'm thinking about carrying on."
Underwood's efforts have been rewarded with more than £4,000 in sponsorship donated to Help For Heroes - and now he has set himself a new target.
"I am very thankful for the support - from everyone who has already made a donation and also the great support we all had on the day.
"The total of donations has gone past £4,100 and now I'm aiming to reach £5,000 before I close it.
"If any supporters would like to help then there is still time to do so through the website. Donations can benefit from Gift Aid too, which all helps, and the money is going to a great cause."
To assist Rory and Help For Heroes you can make a donation by clicking here
