The 55-year-old lawyer completed the event in a time of 5 hours and 52 minutes as well as receiving an anonymous donation of £10,000.
Austin was running for St Margaret's of Scotland Hospice and had already surpassed his £2000 target when he received the massive contribution.
He said: "Through my Just Giving page I had raised more than £2000, but I received a call from an elderly gentleman and he asked how much he could give.
"I said anything was welcome, and he asked me if £10,000 was enough.
"Things like this do restore your faith in humanity."
Austin admitted the course was much tougher than he had anticipated, after initially keeping the pace with the race leaders.
He said: "The first half of the race I was tagging the pacers, who finisher in about 4 hours 30 minutes, but there is a point in the marathon where you start to go up hill.
"Unfortunately, that's where I started to slow down. I would have liked to have been closer to 5 hours, but it was tougher than I imagined."
Austin ultimately found the whole event to be a great experience, with his wife, friends and the cheering crowd watching on.
He said: "At the end, my wife was waiting and I suddenly found the strength to run the final 300 yards.
"I would do it again, although my knees may be telling me no right now."