I have ran my entire life and I still run a few km's most days. Bangkok is the worst city I have ever run in if it's street running that we are comparing, even central Manila was better for running.
Running in the parks where I live in Bangkok is good though because there is so many paths available, even though I think park running is boring.
I might be wrong but midnight marathons in BKK to beat the heat, pollution and vehicle traffic hardly enhances Thailands reputation as a marathon destination. Marathons around Phuket or Samui might be more appealing. I didn't include Chiang Mai due to the seasonal burning pollution, though at the right time of year CM would be a good location.