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 :)