Scott Overall was confirmed on Team GB's marathon squad last year but two men's spots are still available - and will be decided at this weekend's Virgin London Marathon.
Benedict Whitby, Lee Merrien and Andi Jones are going for the two hour, 12 minute qualifying time and Overall is acting as their pacemaker.
Whitby, who works for the City of London police, knows that is a big improvement on his two hour, 15 minute and nine second personal best but he remains upbeat about his chances.
"Scott and I train together and I was with him in Berlin when he ran the qualifying time last year," said Whitby.
"It was a bittersweet moment for me because I was disappointed with how I ran but so pleased for him.
"It will be good to have him there this weekend but he is still learning the event, he has only run one marathon.
"However, having someone you know and trust to set the pace is going to be really beneficial. He's in great form and will do everything he can for us."
Jones and Merrien must also shatter personal bests to secure the standard and pacemaker Overall is desperate for company this summer, with the men's marathon one of the final events on the schedule.
"I don't think I've ever been in such good shape," said Merrien.
"I've been training at altitude in Kenya and it's been really beneficial. It's the mecca for marathon running and all the top guys are either from Kenya or training there.
"We all know the time we are shooting for and we are all pushing each other on."
If no-one achieves the A standard this weekend, UK Athletics might still use their host nation status to select one additional runner with the two hour, 16 minute B standard - which Whitby, Merrien and Jones have all secured.
UK Athletics and British Olympic Association officials will confirm the remaining members of their team on Monday.