With apologies to ChrisL.(for mentioning rain)
I went out yesterday in the Scottish summer to cycle around both of the bridges on the Forth, a round trip of about 40miles. My outward leg was into the slight wind and it was raining lightly, but at about 16 miles (an hour or so) the rain thumped down. i took shelter for 20 mins but realising the rain was on for the duration, continued my ride.
At times the rain was so heavy that I couldn't see the road surface under the flowing water and I had to cycle very carefully as my cycling glasses were almost always obscured.
It was also cold and I was having difficulty keeping my legs warm (didn't want to put longs on because they would have got waterlogged) and by the time I got home (2hrs 20 for the 40 miles) my hips and left knee were twinging a bit.
Today I couldn't commute because my left knee is still a bit sore.
I cycle distances of twice what I did with little problem. Could the pain be as a result of the rain preventing me getting up to temperature for my muscles and joints, and me then pushing too hard, given that?
BTW, Scotland is a beautiful place to visit, but it has been a bit wet of late. Don't let that put you off.