For the last two years, this stretch of the capital's highway system has marked the finish of the event for Staines in his capacity as a paid pacemaker for established marathon runners. Each time he has stepped out of the race having done the perfect job - last April he hit his half-way target of 631/2 minutes to the second.
But now the 32-year-old Belgrave Harrier is planning to go the full distance in what will be only the second marathon of his career. If his third experience of the London event runs to plan like the first two, he will establish his credentials as a bona fide marathon runner and challenge for the remaining Olympic marathon place on offer to the highest-placed Briton.
Like Richard Nerurkar, Paul Evans and the 1993 London winner Eamonn Martin, Staines has come to marathon running after a successful career over 10,000 metres.
He can look back on highs which include a silver medal behind Salvatore Antibo in the 1990 European Championships, and a British 10-mile record in 1993. The lows include an Achilles tendon problem which kept him out of action throughout 1992, and a problem with asthma which ruined his appearance in the 1989 World Cup and makes sporadic impacts on his racing and training.
His coach, Alan Storey, is well acquainted with the problem. "If the air quality is bad, Gary suffers," he said. "There have been a number of Tuesday nights when he has galloped round the track at Kingston sounding like a wounded pig. He makes so much noise because he can't breathe properly."
Staines has only run one previous marathon. It was in April of last year, soon after his second London pacing job; and it was not a happy occasion. Having selected a flat course in Vienna, he competed during a heatwave and missed a crucial drinks station, eventually finishing fourth in 2hr 16min 04sec.
His memories of London, though, are far more positive. In 1994 his job was to keep Eamonn Martin in touch with the leading bunch, and after a mid-race breakaway he found himself carrying on to 15 miles to bring Martin back into contact. "At that point I still felt great," he recalled. "I had trained to go 26 miles. You have to do that. It's no good thinking to yourself, `I'm only doing half, I'll have a big fry-up breakfast and run round in my trainers.' "
As spectators urged him to carry on, he confessed to momentary feelings of doubt. "I was slightly torn," he admitted. "But I didn't know if some disaster might happen around 19 or 20 miles which would put me out of action for months."
Since missing last month's world cross-country trials because of an ankle injury he has been running 140 miles a week, and won the Reading half- marathon last weekend in 63:31 with plenty to spare.
"There is no doubt that Gary is pretty close to the top of the tree in British half-marathon running," Storey said. "The question which should be answered on 21 April is whether he can transfer that ability to the longer event. He is obviously capable of running significantly quicker than he did in Vienna. Whether that means 2.08 or 2.12 remains to be seen."
Air quality permitting, Staines is hoping to make a real impact. "This time," he said, "I'll be able to go round with out getting neck ache through making sure that all my little ducks are all right."