Now one week on, 11-time marathon runner Nisbet, who lives in Shildon in County Durham, is still in agony.
She told MailOnline her family and friends were "shocked" and "didn't believe blisters like that could occur from sunburn".
But the charity worker and photographer said she took every precaution before getting cooked in the sun.
"I was wearing sun cream at the start of the race but hadn't reapplied - sweat and water had pretty much got rid of what I had on the backs of my legs," she added, explaining the temperature at its hottest was about 30C.
Discussing her ordeal online, she tagged posts with "#idiot" and "#sadv.
But she has taken the bad luck in her stride.
At one point she wrote: "Once the next lot of orange pus has been drained from my legs I'll probably lose another 4lbs!"
She added: "This isn't the end of the world. They're only scars. I'm sure I'll live to run another day."