Medical staff treated the man, aged in his 30s, who was on target for a sub one hour 45 run when he became unwell.
One runner who saw the man laying flat on his back near Island Road on the A33 in sight of the finishing line at the Madejski Stadium.
The competitor said: "There were lots of medical people attending to him.
"He didn't look well at all."
Other runners were seen stretching out tired and cramping muscles in the town centre before the dreaded incline of Russell Street.
Medical staff were on hand to give worn-out people oxygen, and several runners were seen being taken to the medical tents trolleys.
Another man was being treated for nausea and being faint in the town centre after the eight mile mark.
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