I've read online not to wear underwear with bike shorts. Unfortunately, I don't have cycling specific attire. 
What would be a best clothing to wear (below the waist, specifically)  for first time 75 km ride on the road? 
Riding "Ride For Heart" in Toronto this Sunday, using Norco Cyclocross bike. 

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**Update:**
Thank you all for the advice. The ride went well and we finished in about 3.5 - 4 hours.  Avg Speed was ~20 km/h. Highest recorded - 37.5 km/h :) 
Used normal synthetic gym shorts with synthetic undies. No rash, but my butt hurts a bit today :)