I think with flat pedals getting a grippy soled pair of trainers is the secret. I have one pair of trainers that grip the pedals like glue in all weathers another similar pair are hopeless.
It may be coincidental but a nagging knee pain I used to get after all day rides has dissapeared since I went back to flat pedals.
I still have the shoes ,cleats and pedals gathering dust in my wardrobe but at the moment at least I dont see me going back to them.
Totally agree with the bad knee thing. A few years ago a did some road biking and every time I did 20+ miles the inner sides of both knees would be in agony and I would be that way for 4/5 days. Tried cleats in different positions and even got pro from lbs to fit me to bike shoes correctly, but made no difference.
I was training for 200 miles in 2 days which I did, but haven't touched road bikes since because of what it did to my knees (couldn't walk properly for weeks!)
I use flats on my mtb and have never had an issue.
I don't for a second think this is because clipless are evil, my bony knees are just not made for them.
Am doing L2P in July and will be using decent flats with a pair of grippy mtb style shoes (on road bike).
Anyone else done long rides using flats? If so how did you get on?