I had the same problem Friday. Riding 15 miles into a head wind and taking decongestants wore me down.
I told the two other riders that I was going home. We stopped and got refreshed. Then headed home.
No wind now, I was in front @17 mph. Made a right turn ,with a tail wind, I could hold 20 mph.
Ended with 30 miles, 55 miles the day before, and 75 miles three days before that.
Head Winds are Tuff for me. Love the Tail winds.
I have a 34 lb bike and weight 212 lbs.