I am 45 now. When I was about 21 or 22 I had a bout of blurred vision especially after a lengthy jog. It last for about 20 to 30 mins. Over the past 23 or so years I have had this about six to eight occasions, no real pattern to it, always lasting for the same length of time. No pain accompanies these bouts.
We've had other posts from runners with the same problem. Don't think any of them showed any other symptoms, eg., of any demyelinating disease. You could look up Uhthoff's Syndrome, but since it's been twenty years since your last bout, it's probably nothing to worry about.