Kastor - whose high altitude base in Mammoth Lakes, California, is currently under 60 feet of snow - revelled in the conditions to win in 2hr 19min 36sec, making her the fourth fastest woman of all time, but Gebrselassie, Ethiopia's multiple Olympic and world champion, found the cold cramping his calfs and hamstrings shortly after the halfway point as he struggled to adapt his habitual tippy-toe running style to the slippery surface.
Having set out with world record ambitions, Gebrselassie pronounced himself surprised even to have finished a race won by Kenya's Felix Limo, who sprinted away from his compatriot Martin Lel, the defending champion, 200 metres from the finishing line in The Mall.
The 33-year-old Ethiopian had set world marks for the half-marathon and 25km in the previous three months and his odds to win had shortened with the late withdrawal of Kenya's world record holder Paul Tergat. Everything seemed on course for him as the leading group went through the halfway point in 62min 33sec, which was 25 seconds inside world-record pace.
That level of performance dropped dramatically soon afterwards as the pacemakers dropped out, leaving seven runners warily eyeing each other and all unwilling to take the race on. But when Lel put in a burst of speed after 23 miles, Gebrselassie could not respond and he eventually trailed home ninth in 2hr 9min 5sec, almost three minutes outside his personal best.
"I was surprised to finish the race," said Gebrselassie. "After 25k it was all over for me, my calf muscles, everything... You have good days, sometimes bad. For me this was the worst day since I started out competing internationally in 1991. But what can you do? If I didn't accept this, how can I accept when I win?
"I tried to train in the rain in Addis Ababa," he added. "That night I had a lot of problems with my calfs and hamstring. It means I have to change a lot of things before I can become a good marathon runner."
Gebrselassie will run some races on the track, including an attempt on the world one-hour record in Hengelo, before returning to the roads for an autumn marathon.
Kastor, in contrast, produced a race of spooky efficiency, with her first and second halfs being run in an identical time. Having won the Chicago Marathon seven months ago, the Olympic bronze medallist claimed her second major title with the kind of assured running London spectators have come to expect from Radcliffe, absent yesterday with a foot injury.
"My hands and hamstrings got a little cold out there, but I would welcome that more than the warmer conditions we were anticipating earlier in the week," she said, having completed the last nine miles on her own. "I had a couple of training sessions in really wet conditions recently - I was well prepared for the rain."
The obvious question - how would she have matched up to Britain's world champion?- was one which Kastor addressed with admirable circumspection. "I would love to race against Paula," she said. "I was very disappointed that she wasn't running in the race. Any time you have the world record holder in an event there is kind of a target on their back.
"I definitely felt after the race that I have improvements I could make," Kastor added. "I didn't feel I maximised my performance today. I definitely like to race against the best and Paula is the best in the marathon. Hopefully we'll meet one day."
That day may come in the autumn at either the New York or the Chicago Marathon. But it may not, as Kastor will not make any decision about which race she will do until August, when she has finished a track season.
The American, meanwhile, is strengthened in her belief that she has the potential to beat Radcliffe. "I would have liked to have done it here in London," she said with a smile. "Although I probably wouldn't have been very well received."
Limo, who has previously won titles in Rotterdam, Berlin and Chicago, added to his growing reputation in marathon running despite having to sidestep an egg that was thrown at him from a flat during the first half of the race.
Britain's Mara Yamauchi was another who finished the race content after coming in sixth in 2hr 25min 13sec, replacing Veronique Marot - the 1989 winner in 2:25.56 - as the second-fastest Briton behind Radcliffe.
All those miles training alone in her adopted city of Tokyo paid off for a runner who spent the second half of the race running by herself.Reuse content