Clothing advice for beginner 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. 



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 :)
I'd invest in a pair of bike shorts.

Casual riders going short distances on big comfy saddles can get away with casual or regular sports clothing. You won’t be able to do that riding significant distance on a sports saddle. 

Bike shorts have padding, obviously, but are also constructed so there is no seam between rider and saddle. Regular sports shorts have a central crotch seam that is painful to ride on. 

The reason to not wear underwear under bike shorts is to not re-introduce seams between rider and saddle.