was out a run with my mate today and ended up going home via Loch Lomond,the first 17 miles were fine as it's a wide road so easy for overtaking,but from Tarbet it was nose to tail (we couldn't even filter ) there must have been a humongous vehicle at the front we couldn't see though as it's a twisty road, we eventually pulled in at the Drovers Inn for a while so the road could clear, of course when we got back on the bike we were like caged animals getting let loose ,when we got home we were both knackered.so don't know if it was because we had to endure riding so slow for ages or what but I'm never usually tired after a run..
As Dilli and Steve said mate. Happens to us all. It just means you need to plan in more frequent bladder stops.